Interferometry systems and methods

ABSTRACT

An interferometry system includes a plurality of coherent light sources that each generate a beam of coherent light. Separate waveguide pathways are optically associated with each coherent light source. Each separate waveguide pathway has an endpoint configured to emit at least a portion of the beam of coherent light from the associated light source. A plurality of photodetectors is optically associated with waveguide pathways. In some cases, a retroreflector receives the light emitted from the endpoints, modulates the received light, and directs the modulated light back to the endpoints. The modulated light and a portion of the coherent light reflected from the endpoint of the waveguide pathway receiving the modulated light is directed a photodetector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/691,932, filed Nov. 22, 2019, and entitled Interferometry Systems andMethods, which is a continuation-in-part of U.S. patent application Ser.No. 16/313,431, filed Dec. 26, 2018, and entitled Interferometry Systemand Associated Methods, which is a 371 National Stage Entry ofInternational Application No. PCT/US2017/039151, filed on Jun. 23, 2017,and entitled Interferometry System and Associated Methods, which claimsthe benefit of and priority to U.S. Provisional Application No.62/354,080, filed on Jun. 23, 2016, each of which is incorporated hereinby reference in its entirety.

BACKGROUND

Interferometry is a measurement technique that involves thesuperimposition of electromagnetic waves. One of the many advantages ofinterferometry includes the ability to achieve measurements withnanometer scale accuracy. Hence, it has been used extensively inmetrology, micro-fabrication, quantum mechanics, and numerous otherfields. Interferometry can also be useful for measuring displacement,rotation, refractive index changes, and numerous other variables.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only illustrated embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 2A is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 2B is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 3 is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 4 is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 5A is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 5B is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 6A is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 6B is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 7A is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 7B is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 8A is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 8B is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

FIG. 8C is a schematic diagram of a system for measuring distance inaccordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics forthe purpose of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailscan be made and are considered to be included herein.

Accordingly, the following embodiments are set forth without any loss ofgenerality to, and without imposing limitations upon, any claims setforth. It is also to be understood that the terminology used herein isfor the purpose of describing particular embodiments only, and is notintended to be limiting. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this disclosurebelongs.

The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments described herein are, for example, capable of operationin sequences other than those illustrated or otherwise described herein.Similarly, if a method is described herein as comprising a series ofsteps, the order of such steps as presented herein is not necessarilythe only order in which such steps may be performed, and certain of thestated steps may possibly be omitted and/or certain other steps notdescribed herein may possibly be added to the method.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments described herein are, for example, capable of operation inother orientations than those illustrated or otherwise described herein.

As used herein, “enhanced,” “improved,” “performance-enhanced,”“upgraded,” and the like, when used in connection with the descriptionof a device or process, refers to a characteristic of the device orprocess that provides measurably better form or function as compared topreviously known devices or processes. This applies both to the form andfunction of individual components in a device or process, as well as tosuch devices or processes as a whole.

As used herein, “coupled” refers to a relationship of connection orattachment between one item and another item, and includes relationshipsof either direct or indirect connection or attachment. Any number ofitems can be coupled, such as materials, components, structures, layers,devices, objects, etc.

As used herein, “directly coupled” refers to a relationship of physicalconnection or attachment between one item and another item where theitems have at least one point of direct physical contact or otherwisetouch one another. For example, when one layer of material is depositedon or against another layer of material, the layers can be said to bedirectly coupled.

As used herein, “associated with” refers to a relationship between oneitem, property, or event and another item, property, or event. Forexample, such a relationship can be a relationship of communication.Additionally, such a relationship can be a relationship of coupling,including direct, indirect, electrical, optical, or physical coupling.Furthermore, such a relationship can be a relationship of timing.

Objects or structures described herein as being “adjacent” to each othermay be in physical contact with each other, in close proximity to eachother, or in the same general region or area as each other, asappropriate for the context in which the phrase is used.

In this application, “comprises,” “comprising,” “containing” and“having” and the like can have the meaning ascribed to them in U.S.Patent law and can mean “includes,” “including,” and the like, and aregenerally interpreted to be open ended terms. The terms “consisting of”or “consists of” are closed terms, and include only the components,structures, steps, or the like specifically listed in conjunction withsuch terms, as well as that which is in accordance with U.S. Patent law.“Consisting essentially of” or “consists essentially of” have themeaning generally ascribed to them by U.S. Patent law. In particular,such terms are generally closed terms, with the exception of allowinginclusion of additional items, materials, components, steps, orelements, that do not materially affect the basic and novelcharacteristics or function of the item(s) used in connection therewith.For example, trace elements present in a composition, but not affectingthe composition's nature or characteristics would be permissible ifpresent under the “consisting essentially of” language, even though notexpressly recited in a list of items following such terminology. Whenusing an open-ended term, like “comprising” or “including,” it isunderstood that direct support should be afforded also to “consistingessentially of” language as well as “consisting of” language as ifstated explicitly, and vice versa.

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result. For example, a composition that is“substantially free of” particles would either completely lackparticles, or so nearly completely lack particles that the effect wouldbe the same as if it completely lacked particles. In other words, acomposition that is “substantially free of” an ingredient or element maystill actually contain such item as long as there is no measurableeffect thereof.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint. However, it is to beunderstood that even when the term “about” is used in the presentspecification in connection with a specific numerical value, thatsupport for the exact numerical value recited apart from the “about”terminology is also provided.

Further, a listing of components, species, or the like in a group isdone for the sake of convenience and that such groups should beinterpreted not only in their entirety, but also as though eachindividual member of the group has been articulated separately andindividually without the other members of the group unless the contextdictates otherwise. This is true of groups contained both in thespecification and claims of this application. Additionally, noindividual member of a group should be construed as a de factoequivalent of any other member of the same group solely based on theirpresentation in a common group without indications to the contrary.

Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to include allthe individual numerical values or sub-ranges encompassed within thatrange as if each numerical value and sub-range is explicitly recited. Asan illustration, a numerical range of “about 1 to about 5” should beinterpreted to include not only the explicitly recited values of about 1to about 5, but also include individual values and sub-ranges within theindicated range. Thus, included in this numerical range are individualvalues such as 2, 3, and 4 and sub-ranges such as from 1-3, from 2-4,and from 3-5, etc., as well as 1, 1.5, 2, 2.3, 3, 3.8, 4, 4.6, 5, and5.1 individually.

This same principle applies to ranges reciting only one numerical valueas a minimum or a maximum. Furthermore, such an interpretation shouldapply regardless of the breadth of the range or the characteristicsbeing described.

Reference throughout this specification to “an example” means that aparticular feature, structure, or characteristic described in connectionwith the example is included in at least one embodiment. Thus,appearances of the phrases “in an example” in various places throughoutthis specification are not necessarily all referring to the sameembodiment.

Example Embodiments

An initial overview of technology embodiments is provided below andspecific technology embodiments are then described in further detail.This initial summary is intended to aid readers in understanding thetechnology more quickly, but is not intended to identify key oressential technological features, nor is it intended to limit the scopeof the claimed subject matter.

The present disclosure relates to novel interferometry devices, systems,and methods for physical measurements in an environment. In oneembodiment, for example, the present technology can be utilized todetect distances between two or more points, in one dimension, twodimensions, and/or three dimensions, depending on the specific designand use of a given device, system, or method. Furthermore, such distancemeasurements can be absolute distance measurements, relative distancemeasurements, or any other measurement between two or more points,including fixed points and moving points. Moving points would thusinclude situations where one point is moving relative to another fixedpoint or to multiple fixed points, as well as situations where two ormore points are moving relative to one another. Thus, the presenttechnology can be utilized to measure location, distance, and changes inlocation and/or distance, to track moving objects, measure velocity,acceleration, deceleration, and the like. Expansion or contraction of anobject that causes variation in the distance between measurement pointscan also be tracked or monitored. Additionally, if the 2D or 3D positionof multiple points on a rigid object are known, then the pitch, yaw, androll of the object can be determined in addition to its 2D or 3Dlocation.

In general, two impinging beams of electromagnetic radiation, that aresufficiently close in frequency, can interfere with one another togenerate heterodyne signals in a detector. Heterodyne signals aresignals resulting from the interference of two or more electromagneticsignals in a non-linear process, like photo detection, for example. Thepresently disclosed technology can accurately determine distance betweentwo or more points from one or more heterodyne signals generated byelectromagnetic waves transmitted through a waveguide or waveguides. Inone embodiment, for example, distance is determined between two (ormore) points by splitting a beam of coherent light into separatecomponent beams, and directing each component beam along a separatewaveguide pathway before recombining them. The coherent light in atleast one of the waveguide pathways is frequency shifted or otherwisemodulated to create heterodyne signals at a photodetector at thedifference frequency (or 2 f). When the light from one waveguide entersa second waveguide that has a light wave of a different frequencypropagating in the same direction, the two frequency components willco-propagate. When they reach the detector, the two beams will produce aheterodyne (beat) signal at the difference frequency in thephotodetector. Such heterodyne signals can be used to measure distance.

In some examples, separate beams of coherent light can be generated froma plurality of coherent light sources, and each separate beam can bepropagated down a separate waveguide pathway. It is additionallycontemplated that, in some examples using such separate light sources,one or more of the separate beams of coherent light can be split intoseparate component beams.

Thus, in some examples, the present interferometry system can include acoherent light source operable to generate a beam of coherent light. Thesystem can further include separate waveguide pathways opticallyassociated with the coherent light source to direct coherent lighttoward a photodetector optically coupled with each waveguide pathway. Atransceiving segment or point can be optically associated with eachwaveguide pathway at a location between the coherent light source andthe photodetector. Each transceiving segment can be configured to emitan emitted beam of coherent light and can be positioned to receive aportion of an emitted beam of coherent light emitted from a transceivingsegment of a different waveguide pathway. The portion of the emittedbeam received from the different waveguide pathway can form part of anoptical interference signal generated from the superposition of beams ofcoherent light. Specifically, the received portion of the emitted beamfrom the different waveguide pathway can be superposed with a portion ofcoherent light from the receiving waveguide pathway, such as a reflectedportion or a portion of the component beam that was not emitted, to formthe optical interference signal. Thus, each waveguide pathway can beconfigured to direct a separate optical interference signal toward arespective photodetector.

In one specific example, an interferometry system can include a coherentlight source, a first photodetector, and a second photodetector.Additionally, a first waveguide pathway can be optically associated withthe coherent light source and operable to emit a first optical beam froma first endpoint (i.e. a first transceiving segment or point). A secondwaveguide pathway can also be optically associated with the coherentlight source and operable to emit a second optical beam from a secondendpoint (i.e. a second transceiving segment or point). Further, a thirdwaveguide pathway can be configured to receive an interfering portion ofthe second optical beam and a reflected portion of the first opticalbeam such that the reflected portion of the first optical beam and theinterfering portion of the second optical beam form a first opticalinterference signal. The third waveguide pathway can be furtherconfigured to guide the first optical interference signal to the firstphotodetector. Similarly, a fourth waveguide pathway can be configuredto receive an interfering portion of the first optical beam and areflected portion of the second optical beam such that the reflectedportion of the second optical beam and the interfering portion of thefirst optical beam form a second optical interference signal. The fourthwaveguide pathway can be further configured to guide the second opticalinterference signal to the second photodetector.

In this example, the first waveguide pathway and the third waveguidepathway can be combined to define a single waveguide pathway from thecoherent light source to the first photodetector. Similarly, the secondwaveguide pathway and the fourth waveguide pathway can be combined todefine a single waveguide pathway from the coherent light source to thesecond photodetector. The first endpoint and the second endpoint alongeach respective pathway can also be defined as first and secondtransceiving points, respectively, positioned along each separatepathway at a position between the coherent light source and therespective photodetector. Any suitable number of such pathways can beemployed in the present system, as will be apparent from thedescriptions of the system provided herein. It is also noted that theterms endpoint and transceiving segment or transceiving point are usedinterchangeably throughout this disclosure. Thus, reference to atransceiving segment or transceiving point can also refer to anendpoint, or vice versa.

It will also be apparent from the present disclosure that theinterferometry systems described herein can be used in a variety ofmethods. For example, a method of determining a distance between aplurality of points can include directing a beam of coherent light alongseparate waveguide pathways toward a photodetector that is opticallyassociated with each separate waveguide pathway. Each waveguide pathwaycan further include a transceiving segment optically associatedtherewith. A beam of coherent light can be emitted from the transceivingsegment in each of the separate waveguide pathways to form an emittedbeam of coherent light. A portion of the emitted beam of coherent lightcan be received at a transceiving segment optically associated with adifferent waveguide pathway from which the emitted beam was emitted. Thereceived portion of the emitted beam of coherent light can form part ofan optical interference signal generated from the superposition of beamsof coherent light. As described above, the received portion of theemitted beam from the different waveguide pathway can be superposed witha portion, such as a reflected portion, of coherent light from thereceiving waveguide pathway to form the optical interference signal.Separate optical interference signals can be delivered to respectivephotodetectors to generate a local interference photocurrent at eachrespective photodetector. A difference between the local interferencephotocurrents at each photodetector can be related to a distance betweenthe transceiving segments of the separate waveguide pathways.

In one specific example, a method of determining a distance between aplurality of points can include emitting a first optical beam from afirst waveguide pathway at a first endpoint or transceiving point andemitting a second optical beam from a second waveguide pathway at asecond endpoint or transceiving point. An interfering portion of thesecond optical beam and a reflected portion of the first optical beamcan be received, combined, or superposed at the first endpoint ortransceiving point. The reflected portion of the first optical beam andthe interfering portion of the second optical beam can form a firstoptical interference signal. Similarly, an interfering portion of thefirst optical beam and a reflected portion of the second optical beamcan be received, combined, or superposed at the second endpoint ortransceiving point. The reflected portion of the second optical beam andthe interfering portion of the first optical beam can form a secondoptical interference signal. The first optical interference signal canbe guided from the first endpoint along the third waveguide to a firstphotodetector to generate a first interference photocurrent. Likewise,the second optical interference signal can be guided from the secondendpoint along the fourth waveguide to a second photodetector togenerate a second interference photocurrent. A difference between thefirst and second interference photocurrents can be related to a distancebetween the plurality of points.

In this particular example, the first waveguide pathway and the thirdwaveguide pathway can be combined to define a single waveguide pathwaydirecting light to the first photodetector. Similarly, the secondwaveguide pathway and the fourth waveguide pathway can be combined todefine a single waveguide pathway directing light to the secondphotodetector. The first endpoint and the second endpoint along eachrespective pathway can also be defined as first and second transceivingpoints, respectively, that are positioned along each separate pathwayand configured to emit a beam of coherent light and positioned toreceive an emitted beam of coherent light from a separate waveguidepathway. Any suitable number of such pathways can be employed in thepresent method, as will be apparent from the descriptions providedherein.

In one specific embodiment shown in FIG. 1, an interferometry device 100can include a coherent light source 102 that generates coherentelectromagnetic radiation at frequency (ω₀) along an initial waveguidepathway 104. The coherent electromagnetic radiation, or coherent light,is split into two waveguide pathways, a first pathway 106 and a secondpathway 108. However, it is noted that the light need not be split inall examples. In some cases, multiple coherent light sources can beemployed to direct coherent light along separate pathways. In specificexamples where the coherent light is split, any suitable splittingconfiguration can be used, such as a beam splitter, pigtailedwaveguides, spliced waveguides, the like, or combinations thereof. Assuch, in some examples, the interferometer system can include a beamsplitter, a waveguide splicer, or the like, to split coherent light intoseparate component beams. It is further noted that the L1-L7 labels inFIG. 1 represent portions of the first and second pathways. The coherentlight in the first pathway has a frequency (ω₁), and the coherent lightin the second pathway 108 has a frequency (ω₂), which is frequencyshifted or otherwise modulated with respect to the other pathway interms of any property of coherent light that can be shifted or modulatedto facilitate distance measurements. Non-limiting examples can includemodulating phase, frequency, amplitude, or any combination thereof. Suchtechniques of modulation are well known in the art, and any such deviceis contemplated. The coherent light in one or more pathways can beshifted or modulated, while in other examples the coherent light in oneor more pathways is not shifted or modulated. As an example, FIG. 1shows elements 110 and 111 located along each of the waveguide pathways,which can include shifters or modulators of light. These elements caninclude any device that is capable of shifting or modulating light in amanner that allows distance measurement according to the presenttechnology, such as frequency shifters, phase modulators, amplitudemodulators, and the like, including combinations thereof. Furthermore,the shifter or modulator element in each pathway can be the same ordifferent as in other pathways. In some examples, one pathway mayinclude an element, while another pathway may not.

The first pathway light at frequency ω₁ and the second pathway light atfrequency ω₂ continue along their respective pathways 106, 108 towaveguide endpoints 112, where light at frequency ω₁ and light atfrequency ω₂ exit their respective pathways. Light at ω₁ enters endpoint112 of the second pathway 108 and light ω₂ enters endpoint 112 of thefirst pathway 106. Additionally, a portion of the light in each pathwayis not emitted at endpoint 112, but is reflected back from endpoint 112to continue along its initial pathway. In some cases, such non-emittedlight can also be transmitted from L1 to L3 (or from L4 to L6) prior toreaching endpoint 112. As will be recognized by one skilled in the art,the index of refraction of the waveguide can be different from the indexof refraction of the air outside the waveguide. As such, when thecoherent light traveling along pathways 106, 108 reach their respectiveendpoints 112, the difference in refractive index between the waveguideand the external air will cause a portion of the light in each pathwayto be emitted from endpoints 112, and a portion to be reflected back inthe original pathway. Thus, the combination of the non-emitted coherentlight with the coherent light received from the other pathway canproduce a superposition of two frequency components at ω₁ and ω₂ in eachof the pathways propagating toward the corresponding photodetector 116,118. Part of this light propagates toward the photodetectors throughpathways (106, 108). The superposition of the two waves produceheterodyne (beat) signals at each photodetector at the differencefrequency (ω₁−ω₂), thus generating respective photocurrents. Thephotocurrents can then be detected and used to calculate the heterodynesignal phase at each photodetector 116, 118, which are in turn used todetermine the distance between the endpoints 112 of the two pathways, asis exemplified below.

The coherent light source can include any light generation device orsystem capable of introducing coherent light into a waveguide, such as,for example, an optical fiber. Non-limiting examples can include fiberlasers, solid state lasers, gas lasers, semiconductor lasers such aslaser diodes, photonic crystal lasers, and the like, includingappropriate combinations thereof. In one specific aspect, the lightsource can be a pigtailed laser diode.

Any output power can be employed that is suitable for use with the othercomponents of the system, such as the photodetectors. Depending upon theapplication, output powers can range from 1 microwatt to more than 1watt. In many applications, however, a 1-100 milliwatt power can beused. The output power can be chosen based upon a variety of criteria,such as the desired signal to noise ratio, detection bandwidth,saturation of the linear response of the photodetectors, and any lightpower safety issues related to the use of the interferometer in aparticular environment. In some examples, it can be desirable tomaximize the output power of the coherent light source withoutcompromising the photodetector or photocurrent generated at thephotodetector so as to maximize both the lateral and longitudinalmeasurement ranges of the divergent beam emitted from the optical fiber.

Further, any suitable wavelength of coherent light can be used in thecurrent system and methods. However, as will be apparent to one skilledin the art, coherent light in the infrared and visible ranges can have anumber of practical advantages. Thus, in one aspect, the light source ofthe current system can emit coherent light having a wavelength of from400 nm to 1000 nm and higher. In some aspects, the light source can emitcoherent light in the infrared range (i.e. having a wavelength from 750nm to 1000 nm or higher, including both IR and near IR ranges). In someaspects, the light source can emit coherent light in the visible range(i.e. having a wavelength from 400 nm to 750 nm). However, differentlight sources can have different limitations with respect to thecoherency of the light they emit (i.e. over longer distances the abilityto interfere can diminish). Thus, some light sources may not be suitablefor all applications of the current systems and methods (i.e.measurements over longer distances) if the frequency range for the lightsource is not sufficiently narrow to generate an adequately coherentbeam to produce an interference signal over the required distances. Assuch, frequency, power, source, and other considerations can be variedto account for the system design and the intended use. The principlesdescribed here may also be applied to electromagnetic waves of otherfrequencies and wavelengths, including microwaves, UV light, and radiowaves.

It is noted that coherent light is delivered through the optical fiber;however, a light signal that is emitted from a waveguide endpoint,superposed or added to a second light signal reflected or reintroducedback into a waveguide may not coherently interfere, depending on thesize of the waveguide, the size of the waveguide outlet, the distancebetween the waveguide endpoints, and the like. While there needs to besufficient coherence in the light to generate interference, the term“coherent” can include both spatial and temporal components. Both ofthese components are generally needed to generate the interferenceeffects described. Both waveguide endpoints that receive transmittedlight from the opposite fiber channel or pathway should also utilizelight that is not orthogonally polarized relative to the opposite fiberchannel or pathway in order for the interference to occur. In thisregard, polarization preserving waveguides or polarizing elements can beuseful.

The waveguide can be any material capable of containing and transmittingcoherent electromagnetic radiation along its length. Optical fibers, forexample, can be generally flexible, and can have minimal mass. A varietyof materials can be utilized as optical fiber materials including,without limitation, silica, transparent polymers, and the like,including appropriate combinations thereof. In one embodiment, apolarization preserving fiber can be used to preserve the polarizationof a signal within the fiber. Furthermore, in some aspects opticalfibers can include single mode fibers. In other aspects, light can bedelivered without using optical fibers, and as such, any mechanism fordelivering light that allows for interference to occur in thephotodetectors is considered to be within the present scope. As oneexample, in some aspects bulk optical devices can be used to deliverlight. However, due to the many practical advantages of single-modeoptical fibers, specific reference will be made to this type ofwaveguide.

A variety of single-mode fibers can be used in the current systems andmethods. In some aspects, single-mode fibers can be selected based ontheir numerical aperture. The numerical aperture of the single-modeoptical fiber can control the angle at which the coherent beam of lightwill emerge from the fiber, thus controlling the lateral andlongitudinal ranges of the light emitted from endpoints of an opticalfiber. For example, where it is desirable to emit the beam from thefiber end at a large angle (i.e. large lateral range), a fiber with alarge numerical aperture can be selected. Conversely, where it isdesirable to emit the beam from the fiber end at a narrow angle (i.e.large longitudinal range), a fiber with a small numerical aperture canbe selected. Accordingly, the numerical aperture of the single-modeoptical fiber can be selected based on a desire for greater lateral orlongitudinal coverage of the coherent optical beam. Generally,increasing lateral coverage (i.e. the beam divergence), can compromisethe longitudinal range of a detectable signal. Conversely, increasingthe longitudinal range (i.e., the distance over which an optical signalcan be detected) can generally compromise the lateral coverage orbreadth over which the optical signal will be detectable. Note thatoptical elements can be positioned near the waveguide ends which canincrease or decrease the effective divergence/acceptance angle of thefiber.

Accordingly, the current system can be used to measure a range ofdistances depending on the longitudinal and lateral coverage of thesystem. For example, for one-dimensional measurements on a single axis,the range of the current system can be large (10 meters, or evenlarger). In this geometry, the coherent beams emitted out of the fiberscan be collimated with a lens or made nearly collimated rather thandiverging, as will be discussed more fully below. However, whenthree-dimensional position measurements are desired, the reference andsignal beams can be diverging and, therefore, at greater distances thepower falling on an optical fiber endpoint of a given pathway fromanother fiber can become smaller. Under these conditions, the noise candetermine the maximum longitudinal and lateral distance that can bemeasured.

As a non-limiting example for illustrative purposes only, if light of633 nm wavelength emitted from a fiber endpoint has a Gaussian shapewith a beam waist of 2×10{circumflex over ( )}-6 meters, then thedivergence half angle will be of order 0.1 radian. This angular widthcorresponds to an approximate effective light emission and acceptancearea of the fiber of order 4×10{circumflex over ( )}-12 meters². In thisexample, it is also assumed that the Gaussian beam has a power of 1 mW.The Gaussian beam will spread as it propagates toward another waveguide(with similar effective acceptance/emission area), and then some of theoptical power from the Gaussian beam will enter the other waveguide andbe combined with light in the other waveguide that is reflected backinto the other waveguide by the fiber end (i.e. due to the difference inrefractive index between the waveguide and the external environment).The two components will co-propagate down the waveguide until they reachthe photodetector, at which point they will produce a heterodyne signalin the photodetector. It is noted that the Gaussian beam light intensityfrom the emitting waveguide will drop as it spreads out toward the otherwaveguide. Assuming that a 6.3 nm resolution distance measurement isdesired, requiring a photodetector heterodyne signal current signal tonoise ratio of 100, the power received by the receiving waveguide (withsimilar effective photodetector area=4×10{circumflex over ( )}-12 meter)can be 4.8×10{circumflex over ( )}-12 watts (based upon calculated shotnoise and a detection bandwidth of 1 kHz under conditions consistentwith this example, as shown in example 1 below). The maximum area thatthe signal beam will have after spreading and reaching the other fiberto produce this signal to noise ratio can be approximately8×10{circumflex over ( )}4 meters², as determined by the followingrelationship: Maximum signal beam area=(signal power/minimum detectedpower)*(photodetector area)=(1×10{circumflex over ( )}-3Watt/4.8×10{circumflex over ( )}-12 Watt)*(4×10{circumflex over ( )}12m²)=8×10{circumflex over ( )}-4 m². In this example, this maximum beamarea corresponds to an approximate beam width of approximately 2.8 cm.Thus, at a divergence angle of 0.1 radian, the beam can spread to aradius of approximately 2.8 cm and still be detected with a signal tonoise ratio of 100 by a detector with an effective area of4×10{circumflex over ( )}-12 meters². This means that the two fiber endscan be laterally shifted by 2.8 cm without significant reduction indistance measurement resolution (6 nm). At this divergence angle (0.1radian), the maximum longitudinal range of the measurement isapproximately 28 cm. If the power is increased, or the bandwidthdecreased or the divergence angle decreased, the longitudinal range canalso be increased. Where the divergence angle is 0.01 radian,corresponding to a Gaussian beam waist of approximately 2×10{circumflexover ( )}-5 meters (effective detector area=4×10{circumflex over( )}-10, then the longitudinal range will increase to approximately 28meters (assuming a 1 mW power, signal to noise ratio of 100), with alateral range of approximately 28 cm.

With proper optics, the divergence angle of the fibers can be madelarger or smaller. This can provide a significant amount of flexibilityfor performing 1D, 2D, and 3D distance measurements. For 1Dmeasurements, precise alignment of two fiber endpoints is not necessary,because measurements can be performed even if the endpoints are notlaterally aligned to better than 2.8 cm (with respect to the firstnon-limiting example).

Continuing the non-limiting example, where several reference fiberendpoints are placed in a reference plane, which in some examples is ata fixed position, and at a separation of less than 2.8 cm between them,where each reference fiber endpoint is configured to separately measurethe distance between that reference fiber endpoint and a signal fiberendpoint, then measurement of the three dimensional position of thesignal fiber endpoint can be detected over a large lateral range (muchlarger than the individual lateral range of each endpoint pair (2.8 cm),using triangulation methods. In some further examples, a plurality ofsignal fiber endpoints can also be used with the plurality of referencefiber endpoints. In some examples, the number of reference fiberendpoints can exceed the number of signal fiber endpoints.

Suitable ranges can be ranges at which a fiber/photodetector (as shownin FIG. 1) can detect an interfering optical signal from an associatedfiber/photodetector. As shown above, the minimum detectible power of theinterfering beams can be determined by the noise which dominates in thedetection process and the bandwidth of the detection system. As is shownbelow, a heterodyne interference signal detected by a photodetector isproportional to the square root of the product of the power of the twointerfering waves. The light wave which is reflected from the endpointof the waveguide is typically very strong compared to the beam which istransmitted from one waveguide and enters the other. Under theseconditions, the heterodyne power can be much larger than the power ofthe transmitted wave by itself, making it possible to detect very smallamounts of light transmitted between the two fibers.

As previously discussed, an optical fiber can be a single optical fiberor split into separate fiber channels, thus splitting the beam ofcoherent light into separate component beams and directing eachcomponent beam along a separate waveguide pathway. A single or multiplefiber channels can include a frequency shifter, phase modulator, opticalmodulator, or the like. Nonlimiting examples of phase modulators oroptical modulators can include an acousto-optic modulator, anelectro-optic modulator, a magneto-optic modulator, a mechano-opticmodulator, a phase modulator, or other suitable device. In one example,a frequency shifter can be used. In another example, a phase-modulatorcan be used. In yet another example, an acousto-optic modulator can beused. As such, coherent light is delivered into at least two pathwaychannels, and is frequency shifted or modulated via frequency or phasemodulation devices in at least one channel such that the light in eachseparate channel has a different frequency (f₁, f₂, f₃ . . . ). It isnoted that various types of modulation can be utilized, includingwithout limitation, frequency modulation, phase modulation, amplitudemodulation, frequency shifting, phase shifting, and the like, includingcombinations thereof. By using a different modulating frequency in eachchannel, interference between any pair of channels can be independentlydetected by measuring the heterodyne signal at the difference frequencyof the two channels.

Any type or design of photodetector can be used, such as, for example, aphotodiode having a p-n junction or a p-i-n junction. As will beappreciated by one skilled in the art, many suitable variations oralternatives can be employed to select and/or prepare a suitablephotodetector.

The photodetector is positioned so as to receive the two co-propagatingwaves in the waveguide at ω₁ and ω₂ (heterodyne signal) from theassociated channel, which can be accomplished using a number ofconfigurations. For example, in one embodiment the photodetector can bepigtailed directly to the waveguide channel. In another case, the lightwaves emitted from the waveguide channel can be focused on thephotodetector using a lens or other similar optical device.

A variety of lenses can be used at any useful location along any of thewaveguide channels, such as at the waveguide endpoints (FIG. 1, 112) orat the waveguide channel/photodetector interface, for example. Dependingon a particular application, such a lens can function to change theangle of divergence, to collimate and/or refocus the light signal at aspecific location, or the like. As non-limiting examples, graded indexlenses or regular lenses can be used. Thus, similar to the numericalaperture of the optical fiber, the lens can also be used to control thebeam divergence and the associated longitudinal or lateral coverage ofthe beam.

As previously noted, at least one waveguide channel can be directionallyoriented toward another waveguide channel such that light emitted fromthe waveguide endpoints will impinge upon an opposite-facing waveguidechannel endpoint. Heterodyne signals generated thereby are picked up bythe waveguide endpoints, and therefore a given photodetector receives atleast a component of light from both waveguide channels. Once at thephotodetector, each heterodyne signal generates charge carriers in theassociated photodetector related to the optical field variations in thatsignal. The electrical signal that is output from a given photodetectoris an electronic representation of the heterodyne signal, which can beused to determine the distance between the waveguide endpoints (e.g.FIG. 1, 112). One parameter that can be useful is the phase of theheterodyne signal, which can be detected using a lock-in amplifier orother suitable device. It can be shown that by measuring the phase ofthe heterodyne signal at each photodetector, and comparing these phases(taking their difference), the optical path length (phase delay)experienced by the light while in each of the waveguide pathways can beeliminated. Simultaneously, the difference phase between the heterodynesignals from the two detectors can be directly related to the distancebetween the waveguide endpoints or changes in the distance between thewaveguide endpoints.

Various techniques can be utilized to detect phase, and while lock-inamplifiers are specifically described, such should not be seen aslimiting as the present scope includes other known phase detectiontechniques. As illustrated in FIG. 2A, one example of a lock-inamplifier 202 can be used to detect the photocurrents from eachphotodetector 204, 206. Respective photocurrents generated atphotodetectors 204 and 206 can be amplified via amplifiers 208 a,b.Optional filters 210 a,b can be used to filter the signals before goinginto the lock-in amplifier 202. However, in many cases a lock-inamplifier can provide adequate signal filtering without the addition ofsupplemental filters. In this case, the heterodyne signal from onedetector is used as the reference signal to the two channel lock-inamplifier which detects the signal from the other detector. Thus, alock-in amplifier can be used to reliably detect and extract the phasedifference between the heterodyne signal coming from each of the twophotodetectors. Alternatively, as illustrated in FIG. 2B, a frequencysignal from an optical modulator 220 can be sent as a reference signalto the lock-in amplifier 202, detecting the phase of the heterodynesignal from a detector with respect to the electrical signal used toproduce the optical modulation in that waveguide. FIG. 2B illustrates asignal coming from a single optical modulator and waveguide.

As is described in more detail below, the interference photocurrent fromeach of the photodetectors can be used to determine the distance orchange in distance between the waveguide endpoints. It is noteworthythat the two heterodyne signals from the two photodetectors are at thesame frequency (difference between the two modulation frequencies of thetwo channels or fiber arms). These can be detected by a single lock-inamplifier, if one signal is input as the reference to the lock-inamplifier and the other as the signal to the lock-in amplifier. It isalso possible to use two separate lock-in amplifiers, each detecting theheterodyne signal from each photodetector separately, using a lock-inamplifier reference produced directly from the modulators (electricalsignal) at the difference frequency of the two modulators, and thelock-in signal coming directly from a single photodetector. In thisscenario, the phase of each signal can be measured separately, and thenthe difference between the two signals can be determined by subtractingthe two-phase signals from the two lock-in amplifiers.

For example, FIG. 3 shows one embodiment of an electronic configurationfor phase detection where a modulator is used on one pathway. In thiscase, pathway 1 (not shown) delivers light to a first photodetector 302,and pathway 2 (not shown) delivers light to a second photodetector 304via an optical modulator 306. As illustrated, separate lock-inamplifiers 308 a,b can each detect the heterodyne signal at eachphotodetector 302,304 separately, using a lock-in amplifier referenceproduced directly from the modulator 306 (electrical signal).

FIG. 4 illustrates another electronic embodiment for phase detection,where the light pathway has been removed for clarity. Light is deliveredto photodetectors 402, 404 through optical modulators 406, 408.Electronic signals generated in the photodetectors 402, 404 from theheterodyne signals is delivered to the signal inputs of lock-inamplifiers 410, 412. The electronic difference frequency of the opticalmodulators 406, 408 can be used as a reference signal for the lock-inamplifiers 410, 412. In one example, the optical modulator 406, 408frequencies can be sent to a mixer 414 to obtain f₁+f₂ and f₁−f₂reference signals. A filter 416 filters the reference signal f₁+f₂ andpasses the f₁−f₂ reference signal to the reference input of the lock-inamplifiers 410, 412. In this scenario, the phase of each heterodynesignal can be measured separately, and then the difference between thetwo signals can be determined by subtracting the two phase signals fromthe two lock-in amplifiers.

In some example embodiments, waveguide endpoints can be oriented in acommon direction, resulting in a novel device for measuring distances ina variety of applications. In such a geometry, coherent light emittedfrom a first waveguide endpoint travels to a surface, and is reflectedback, either by the surface itself or a reflector on the surface (e.g. aretro-reflector element in one example). The light from the firstwaveguide then enters the second waveguide and produces a beat signal atthe photodetector of the second waveguide. The same is true for thelight emitted from the second waveguide. It travels to the surface andis reflected back from the surface or a reflector (or retro-reflector).This reflected light then travels and enters the first waveguide whereit produces a beat signal detected by the photodetector of the firstwaveguide. Since the beat signals in both detectors are at the samefrequency, a phase difference between the two beat (heterodyne) signalscan be measured, in the same way as when the two fibers are pointingtoward each other. This system measures the round trip distanceinvolving a reflection from a surface between the two fibers.

As is shown in FIG. 5A, for example, an interferometry device 500 caninclude a coherent light source 502 that generates coherent light alongan initial waveguide pathway 504 (ω₀). The coherent light is split intotwo waveguide pathways, a first pathway 506 (ω₁) and a second pathway508 (ω₂). The coherent light in one of the pathways is shifted orotherwise modulated with respect to the other pathway in terms of anyproperty of coherent light that can be modulated to facilitate distancemeasurements. Non-limiting examples can include modulating phase,frequency, amplitude, or any combination thereof. Such techniques ofmodulation are well known in the art, and any such device iscontemplated. In some examples, the coherent light in both pathways (orin every pathway, in cases of more than two pathways) can be shifted orotherwise modulated. As an example, FIG. 5A shows a light modulator 510,511 located along each of the waveguide pathways. The light modulators510, 511 can be any device that is capable of modulating light in amanner that allows distance measurement according to the presenttechnology, such as frequency shifters, phase shifters, amplitudemodulators, and the like, including combinations thereof. Furthermore,the light modulator in each pathway can be the same or different fromthe light modulator in other pathways. In some examples, one pathway mayinclude a light modulator, while another pathway may not.

Whether modulated or not, the first pathway light ω₁ and the secondpathway light ω₂ continue along their respective pathways 506, 508 towaveguide endpoints 512, where light ω₁ and light ω₂ exit theirrespective pathways. It is noted that, while the first and secondwaveguide pathways can be the same or different lengths and can followthe same or different paths, the waveguide endpoints 512 are positionedadjacent to one another, pointing in the same direction or a directionsuch that the light from one fiber, after reflection can enter the otherwaveguide. Light ω₁ and light ω₂ are emitted from the waveguideendpoints 512 and reflect off of a surface 522 to be measured. A portionof reflected light ω₁ enters the waveguide endpoint 512 of the secondpathway 508, and forms a heterodyne signal on the detector 518 ofwaveguide 2. Similarly, a portion of the reflected light ω₂ enters thewaveguide endpoint 512 of the first pathway 506, and forms a heterodynesignal at the photodetector 516 of waveguide 1, generating therespective photocurrents. The photocurrents can then be used tocalculate the heterodyne signal phase at each photodetector 516, 518,which are in turn used to determine the distance from the waveguideendpoints 512 to the surface 522.

FIG. 5B illustrates a variation of FIG. 5A. Specifically, in the exampledepicted in FIG. 5B, the reflective surface 522 has been replaced with aretroreflector 523 (e.g. a spherical retroreflector or “cat's eye,” forexample). Further, a lens 530 has been positioned proximate theendpoints 512, which are positioned close together (such as within 25micrometers, for example). The lens 530 can be positioned such that thefocal point 532 of the lens 530 can be further from or nearer to thelens 530 than the endpoints 512, which can allow light from pathway 506to illuminate endpoint 512 of pathway 508, and vice versa. This can helpmaximize the amount of coherent light impinging on each of the fiberends.

FIGS. 6A and 6B illustrate some examples of the present system where thefiber ends are not facing one another or facing in a common direction.In some examples, the fiber ends can be positioned orthogonal to oneanother. As is shown in FIG. 6A, for example, an interferometry device600 can direct coherent light along a first pathway 606 (ω₁) towardphotodetector 616 and along a second pathway 608 (ω₂) towardphotodetector 618. The coherent light in one of the pathways can beshifted or otherwise modulated via modulator 610. Any suitable modulatorcan be used, such as described above with respect to FIG. 5A. Prior toreaching photodetectors 616, 618, the first pathway light ω₁ and thesecond pathway light ω₂ continue along their respective pathways 606,608 to waveguide endpoints 612, where light ω₁ and light ω₂ exit theirrespective pathways.

The first pathway light ω₁ and the second pathway light ω₂ can then besplit by a beam splitter 640. In some specific examples, the firstpathway 606 and the second pathway 608 can be optically associated withthe beam splitter 640 at their respective endpoints 612. Whether coupledor not, split potions of the first pathway light ω₁ and the secondpathway light ω₂ can be directed toward a reflective surface, such asretroreflector 623. Optionally, the split potions of the first pathwaylight ω₁ and the second pathway light ω₂ can be focused towardretroreflector 623 using a lens 630. In some examples, the beam splitter640 can be configured to direct the first pathway light ω₁ and thesecond pathway light ω₂ toward the retroreflector 623 without the use ofa lens 630. Light ω₁ and light ω₂ are reflected off of theretroreflector 623 and directed back to the beam splitter 640. In someexamples, a lens 630 can be positioned to focus the reflected light ω₁and reflected light ω₂ toward the beam splitter 640. A portion of thereflected light ω₁ is directed by the beam splitter 640 to enter thewaveguide endpoint 612 of the second pathway 608, which forms aheterodyne signal on the detector 618 of waveguide 2, generating therespective interference photocurrent. Similarly, a portion of thereflected light ω₂ is directed by the beam splitter 640 to enter thewaveguide endpoint 612 of the first pathway 606, which forms aheterodyne signal at the photodetector 616 of waveguide 1, generatingthe respective interference photocurrent. The photocurrents can then beused to calculate the heterodyne signal phase at each photodetector 616,618, which are in turn used to determine the distance from the waveguideendpoints 612 to the retroreflector 623.

FIG. 6B illustrates a variation of the system depicted in FIG. 6A.Specifically, in the example illustrated in FIG. 6B, the beam splitter642 can be a polarizing beam splitter. A quarter wave plate 644 can beused in connection with the polarizing beam splitter 642. In thisspecific example, the polarizing beam splitter 642 can be configured todirect polarized light ω₁ and polarized light ω₂ through the quarterwave plate 644 and onto the retroreflector 623. The polarized light ω₁can be reflected from the retroreflector 623 back through the quarterwave plate 644 and polarizing beam splitter 642 to maximize the amountof light ω₁ that enters fiber endpoint 612 of the second pathway 608,which forms a heterodyne signal on the detector 618 of waveguide 2.Similarly, the polarized light ω₂ can be reflected from theretroreflector 623 back through the quarter wave plate 644 andpolarizing beam splitter 642 to maximize the amount of light ω₂ thatenters fiber endpoint 612 of the first pathway 606, which forms aheterodyne signal on the detector 616 of waveguide 1. The photocurrentscan then be used to calculate the heterodyne signal phase at eachphotodetector 616, 618, which are in turn used to determine the distancefrom the waveguide endpoints 612 to the retroreflector 623.

Numerous applications for such a system of measurement are contemplated,a few non-limiting examples of which are described. The sensitivity of awell-compensated interferometer can be used to detect very small heightor index of refraction changes on a surface. Such a system can scanacross a surface, taking multiple distance measurements to characterizevarious surface features, roughness, refraction changes, and the like.Non-limiting examples can include finger print scanners, thin filmdetectors, defect detectors, molecular film location detection (DNA gelelectrophoresis readout), and the like. Such a system can also be usedto read out a topographic bar code, which would be based on surfaceheight or phase delay of a surface structure. In another example, thesystem can be used as a credit card reader, in which the information isencoded by topography or optical phase. A system using reflection modecan be used for surface profiling, as in a scanning microscope, or anon-contact inspection system, even on rough surfaces. In anotherexample, such a system can be used to detect the motion (defection) ofan Atomic Force Microscope cantilever. When a known physical distancechange is available, it can be used to measure the index of refractionof a medium. Waves or disturbances passing through a known separationbetween the fiber ends can be used to detect and determine the strengthof the wave passing through. Examples would be to measure sound in airor water (hydrophone), or temperature, chemical composition, pressure ofgas or liquid through which the optical beam pass. One example mayinclude measurement of surface acoustic waves, for example, in a surfaceacoustic wave filter.

In another embodiment, the system can be utilized to detect only lightfrom a dynamic spatially localized phase shifter oscillating at a fixedfrequency. In this implementation, the system described above can beused to send light from one fiber (at frequency w1) into the other fiberby reflecting from a surface. Light from the other fiber (at frequencyw2) is sent to the surface and enters the first fiber in a similar way.However, in this case, a reflecting element is placed on the surface,and its vertical position is modulated by an actuator (at frequency w3),such as a piezoelectric film or device. This modulation of its heightwill produce a phase modulation on the reflected beams. Typical phasemodulation amplitudes would be 90 degrees or Pi/2. If the modulationoccurs at a fixed frequency (w3), a beat signal can be obtained atfrequency w1−w2+w3 or w1−w2−w3 at both detectors. The phase of thesebeat signals can be measured and compared to determine movement and orlocation of the localized reflecting phase shifter element at thesurface. The advantage of this system is that only light that is phasemodulated at frequency w3 is detected. Therefore, any light scatteredback into the fibers by the surrounding surface will not contribute tothe detector signal at frequency w1−w2+w3 (or other fixed frequencies).In another example, such measurements can be performed using light atthe same frequency from each fiber (w0). In that case, the phasemodulation at frequency w3 would produce a beat signal in each fiber atfrequency w3 and its harmonics. This would eliminate the need forfrequency shifters in the waveguide pathways.

In some example embodiments, distance can be determined using multiplewaveguides, including the waveguide endpoints, one or more surfaces orlayers of the photodetectors, or other photodetector structures, and thelike, and as such, the locations of where distance measurements aretaken from should not be seen as limiting the present scope. In onespecific aspect, the distance is the distance between the cores of thefiber waveguides. In other cases, distance can be a measure of changesin distance or location, and thus a distance from a reference point to agiven position can be determined without knowing the exact location ofthe waveguide endpoints.

Additionally, 2D or 3D measurements can be made using the principle oftriangulation. For example, a 1D system can be constructed to measurechanges in distance using a single pair of waveguides and detectors. Inthis system, the divergence of the light can make it possible to moveone of the two waveguides laterally with respect to the other over afinite lateral range without losing the interference signal. Thedistance measured by this system will be the changes in absolutedistance between the two waveguide endpoints even when the two endpointsare not aligned on a single axis. The distance measured will be thetotal 3 dimensional distance between the two endpoints.

From this system, a 2D or 3D system can be constructed. This can be doneby having one signal or positioning waveguide endpoint whose 3D positionis of interest to be known, and then a series of other referencewaveguide endpoints that are fixed in a reference grid. These referencedetectors can be arranged such that each acts with the signal waveguideendpoint to uniquely measure the change in absolute 3D distance betweenthe reference waveguide endpoints and the signal waveguide endpoint atany given time. This can be achieved by causing each reference waveguideendpoint to have a unique frequency shift in that pathway that isdifferent from all other reference endpoints, thus enabling theinterference signal from each adjacent pair of reference endpoints to beindependently detected. If the reference endpoints are all laterallyseparated in space and fixed, by measuring the changes in 3D distancebetween the signal waveguide endpoint and all, or a plurality, of thereference waveguide endpoints, triangulation formulas can be used touniquely determine the 3D position of the signal waveguide relative tothe known reference waveguides. Thus, to have 3D knowledge of the signalwaveguide endpoint, at least three reference endpoints can be used. Thelight from a single laser source can be modulated (frequency shift,phase shift, amplitude) in each reference arm at a different frequencyto provide a unique signal to that arm. Initial calibration proceduresof the 3D position of the waveguide endpoints may be needed to establishthe equations to extract the 2D or 3D position of the signal endpoint.Once calibrated, these relationships will enable the 2D or 3D positionto be determined at a rapid rate.

In other example embodiments, systems can be utilized to provide 3Dposition measurements that can include rotation, linear (3D), pitch,yaw, rotation, and straightness, as well as coordinate measurementmachine (CMM) functionality. 3D position measurements of a signal fiberend can be made over a 3D volume under various conditions using at least3 reference fibers and at least 1 signal fiber. If the measureablevolume is larger than what can be accessed by the 3 reference detectorsdue to their finite acceptance angle, then one can use a rotationaldevice to reorient the reference and signal detectors so that they nowfall within the divergence/acceptance angle to perform distancemeasurement. However, since rotation of the reference or signal fibersmay change the 3D location of their end points, a method for determiningthe new position of the fiber ends relative to their positions beforerotation is needed. By having 3 signal fibers (with frequenciesw1,w2,w3) and 3 reference fibers (with frequencies w4,w5,w6), all ableto exchange light between each another, the changes in position of eachfiber can be determined, even during rotation. If all positions areknown initially, then the changes can be constantly monitored todetermine the location at any given time. If the reference fibers(rigidly affixed to a plate, with pre-calibrated 3D locations) arerotated from their initial positions, then by monitoring the distancebetween each fiber end and the 3 signal fiber ends (whose position isknown by previous measurements), then the new 3D positions of thereference detectors can be determined, using standard triangulationmethods. Once the rotation of the reference detectors has occurred andthe new location of the 3 reference fiber ends is known, then the 3signal fibers can be rotated, while monitoring the distance between allof the 3 signal fibers and the 3 reference fibers. With knowledge of theposition of the signal fiber ends before rotation, and the measureddisplacement changes of each fiber measured during the rotation, the new3D location of each of the signal fibers can be determined afterrotation. At this point, the position of all of the 3 signal fibersrelative to the 3 reference fibers is known, so additional relativemovement between the signal and reference fibers can be accuratelymeasured over the additional measurement volume provided by therotational movements. When the position of the 3 signal fibers againstarts to fall outside of the angular range of the signal fibers (andvice versa), the process can be repeated. By this method, a 3Dmeasurement of the signal fibers can be determined over a large 3Dvolume, even when it is required to rotate both the signal and referencefibers to assure that they fall within the divergence/acceptance anglesof the fibers.

In some geometries, it is advantageous to have a large fiber divergenceangle, so that the light diverges quickly and so that light detected canarrive from large angles and still be collected by the fiber. Theacceptance angle of a single mode fiber is determined by the numericalaperture of the fiber, which is set by the size of the fiber mode andindex of refraction difference between the core and the cladding. Byadding various types of optical elements, the numerical aperture can beincreased or decreased. Such elements include an aperture, divergent orconvergent lens, diffraction grating, phase grating, evanescentlycoupled film, diffusing film or a reflector.

Single mode fiber connectors can have a deposition of one or more thinfilms either to create a reflection at the interface or to avoidreflection. This principle can be used to optimize the 3D fiberinterferometer. Fiber ends can be coated to increase or decreasereflection at the interface.

Triangulation methods are well known for determining 3D location. Thesemethods will be used to determine the location of signal and referencefibers. When multiple measurements are performed, additionaldetermination of the pitch, yaw, roll and straightness of a rigid objectis possible, among other degrees of freedom.

When using a fiber interferometer system in which the light is emittedin the same direction from each fiber, it may be necessary to provide ameans for the light returning from the reflective surface to enter intothe other fiber from which it was not emitted. There are many methodsknow in the art to redirect light beams including beam splitters,diffusers, apertures, gratings, defocussed lenses, etc.

Use of polarization preserving fibers may be necessary in order to avoidinadvertent polarization rotation, which could cause the amplitude ofthe beat or heterodyne signal to become small or disappear.

In all measurements discussed here, with a known fixed wavelength oflight, the heterodyne signal phase difference can be related to thedistance (optical path length) the light has traveled between the fiberends. To achieve simple measurements, each cycle of phase delay (calleda fringe) can be counted, corresponding to an optical path length changeof 1 wavelength. These fringes can be counted digitally providing a wayto measure distance. If sub-fringe (sub-wavelength) position resolutionis needed, then rather than counting fringes, the phase difference canbe measured with sub-fringe precision, by directly measuring theabsolute phase difference between the two detectors. In that case,sub-wavelength precision can be achieved in the distance measurement.Achieving distance resolution of 1/1000 of a wavelength or less ispossible using this method.

Multiple fibers pointing in different directions can also be used toavoid the limitation imposed by finite numerical aperture of the fibersused. These fibers could be multiplexed (sending light into them bylight switches only when needed) or could have light emitted from themcontinuously.

Calibration of the interferometer system can be accomplished in a numberof ways. In one example, a separate calibrated positioning system can beused to accurately scan the signal (e.g., movable) fiber (or fiber end)laterally and at different ranges while near reference (fixed) fibers(or fiber ends). During the scans, the distances between the signalfiber (or fiber end) and the reference fibers (or fiber ends) can bemeasured interferometrically using the present methodology. Thisdistance data can then be. fitted to triangulation formulas, which canbe used to uniquely determine the location of the reference (e.g.,fixed) fibers (or fiber ends). With the positions of the referencefibers (or fiber ends) known, the reference fibers (or fiber ends) canthen be used to locate the 3D position of other fibers (or fiber ends)relative to the reference locations. Once calibrated, the location ofthe other fibers (or fiber ends) can be determined without thecalibrated positioning system.

As has been described, the difference in phase between heterodynesignals is used to detect the distance or change in distance between thewaveguide endpoints and to eliminate dependence on the optical pathdifferences in the two fiber arm paths. More specifically, the opticalinterference of the two waves in the photodetectors produce chargecarriers in each, and the resulting photocurrents contain heterodynesignals that can be compared to detect the change in distance betweenthe waveguide endpoints. Without intending to be bound by any scientifictheory, the photocurrents produced in each detector include a static(dc) and a time varying photocurrent. The photocurrent in each detectorcan be sinusoidal or sinusoidal-like, having a frequency of about thefrequency difference 4 f between the optical beams in the two arms ofthe interferometer.

The following describes the optical phase difference Δφ between the twopaths (the optical wave number k₀=2π/λ₀). The description uses the termsfrom FIG. 1. At photodetector A, there will be a sinusoidalphoto-current (heterodyne signal) at the difference frequency (Δf) witha phase as shown by Equation I:

φ_(A) =k[(L ₄ +L ₅ +L ₇ +L ₂ +L ₃)−(L ₁+2L ₂ +L ₃)]  I

At photodetector B, there will be a sinusoidal photo-current (heterodynesignal) at the difference frequency (Δf) with a phase as shown byEquation II:

φ_(B) =k[(L ₄+2L ₅ +L ₆)−(L ₁ +L ₂ +L ₇ +L ₅ +L ₆)]  II

If the phase at each photodetector is measured by, for example, alock-in amplifier, and the difference Δφ between the phases is measured,the result is as shown in Equation III:

Δφ=φ_(A)−φ_(B) =k2L ₇  III

As can be seen, the difference phase Δφ is dependent upon the distanceL7 between the fiber ends and the optical k-vector k, which depends uponthe wavelength of the light used.

One advantage of the presently disclosed technology pertains to the sizeof the fiber waveguide and how such size relates to alignment betweendetectors. As the size of the detector decreases, the acceptance anglewith good optical interference increases. Without wishing to be bound bytheory, the strength of the optical heterodyne signal is proportional tothe square root of the power of each of the two interfering beamsincident on each detector. The photocurrent is proportional to thestrength of the optical heterodyne power as shown in Equation IV:

Optical heterodyne power (at the difference frequency)=2√{square rootover (P ₁ P ₂)}  IV

I=heterodyne current=alpha*optical heterodyne power=2*alpha√{square rootover (P ₁ P ₂)}

-   -   where alpha is the responsivity of the detector.

The magnitude of the heterodyne current I is proportional to the opticalheterodyne signal, which is proportional to the square root of P₁, thepower reflected from the end of waveguide 1 and P₂, the power of thesignal which enters waveguide 1 from the other waveguide Because thereflected power P₁ in waveguide 1 is high, an adequate heterodynephoto-current can be generated even by a low amount of power P₂collected from the divergent beam from the other waveguide.

Because of the small effective size of the waveguide, the acceptance anddivergence angle of the waveguide is larger. Therefore, precisealignment of the signal and reference detectors is not as critical toperform measurements. In other words, a coherent light beam (referencebeam) emitted from a large waveguide must be aligned to a tighterangular tolerance as compared to a smaller diameter beam for effectiveinterference. For example, photodetectors that use visible light (say,0.5 um wavelength as an example) that are approximately 1 mm in diameterwould require an approximate angular alignment tolerance within 4×10⁻⁴radians in order to achieve good (efficient) interference in distancemeasurement.

Photodetectors with an effective size of about 2 microns in size(reference beam widths of about 2 microns), on the other hand, wouldhave an approximate angular tolerance of order 0.1 radians in order toefficiently produce interference. Thus, as the size of the waveguidedecreases, the angular acceptance (and emission angle) toleranceincreases.

It is noteworthy that once two frequency components of light arecombined in a waveguide, they propagate together at the same speed, sothat their phase relationship is constant. Therefore it does not matterwhether they propagate for a long or short distance. When they finallyleave the waveguide and reach a photodetector, the heterodyne signal isthe same as that which would have been produced if the detection hadbeen made at or near the entrance of the fiber carrying the twofrequency components.

Furthermore, without wishing to be bound by theory, the smaller thediameter of the reference beam from each of the fiber channels, thegreater the divergence of the beam can be, thus increasing thedivergence angle of the beam and the corresponding lateral range inwhich the opposite photodetector can be placed to detect interferencesignals. Hence, choice of optical fiber can be based on the size of thefiber core. Single-mode fibers can have a core with a diameter less than15 microns or less than 10 microns or approximately 1-2 microns.Single-mode fibers are preferred due to their small core size and thecorresponding smaller diameter incidence beam that will increase thediffraction angle of the light emitted and therefore the lateraldetection range of the opposite small photodetector. It is also criticalthat single-mode fibers be used to maintain the phase relationshipbetween the propagating frequency components in the waveguide. Thephotodetectors in this method do not have to be specialized.

Additionally, because the incident beam can be very small and thedivergence angle can be large, two or three dimensional measurements canbe made using two, three or more detectors. The interference signalsdetected by each of the detectors can be used to measure the distancebetween each of the detectors relative to the other detectors andtriangulate the position of any one of the photodetectors relative tothe others in three dimensions.

The small size and weight of the fiber elements and the correspondingbroad detection ranges and angles can allow the current technology to beapplied in a large variety of devices and applications. One of theadvantages of the current technology is that it does not require thesophisticated equipment and time constraints for highly accurate beamalignment required by some other interferometric methods. It also usesrelatively inexpensive materials for lower cost production. Hence, thecurrent technology can be useful and desirable in many machines, devicesand services. In some examples, an interferometer system can beoperatively coupled to a machine, such as metrology equipment,manufacturing equipment, robots, vehicles, machining tools and the like.In one aspect, the current technology can be used in metrology toproperly calibrate engineering equipment, measurement equipment, andother equipment. In another aspect, the current technology can be usedfor metrology in micro-fabrication of semiconductors and other similardevices to ensure proper alignment of wafers or other substrates andcomponents, to evaluate surfaces, and to perform any other suitablemicro-fabrication tasks. In one aspect, the multiple fibers can becoupled to robotic arms to accurately position the arms relative to areference frame to grasp or otherwise engage or avoid an object. Therobot arms could be part of a stationary robot or as part of anambulatory robot. The robot could be used in assembly lines, militaryapplications in drones or other equipment, in home consumer products orservices, or for various other products and purposes. The robot couldhave three or more waveguides to give it three dimensional measurementcapability, as previously described. One way to accomplish this would beby using one or more than one waveguides attached to the roboticappendages, or other measurable locations on the robot, each with adetector or set of detectors. Additionally, positional information aboutthe robot or other device can be measured by placing one or morewaveguides in locations away from the robot or other device. Thedistance measurement can also be used to measure angles and othergeometric quantities. The current technology can also be used in avariety of other devices, systems, and methods. It can also measurevelocity, acceleration, etc.

Attention is now directed to FIGS. 7A-8B, which illustrate exampleembodiments that may be used to determine absolute or relative position,three-dimensional orientation, object dimensions, and the like inaccordance with the above-described and additional principles and usingunique arrangements of components.

For instance, FIG. 7A illustrates an example interferometry device 700that includes a plurality of light sources 702 (702 a, 702 b, 702 c)that generate coherent light along a plurality of associated initialwaveguide pathways 704 (704 a, 704 b, 704 c). The initial waveguidepathways 704 (704 a, 704 b, 704 c) direct the light from the lightsources 702 (702 a, 702 b, 702 c) into associated fiber couplers 706(706 a, 706 b, 706 c). In the illustrated embodiment, the fiber couplers706 (706 a, 706 b, 706 c) are 2-by-1 fiber couplers, meanings that eachfiber coupler 706 has two waveguide pathways connected to a first sidethereof and one waveguide pathway connected to a second side thereof.The initial waveguide pathways 704 (704 a, 704 b, 704 c) are connectedto the first sides of the associated fiber couplers 706 (706 a, 706 b,706 c). The second side of each of the fiber couplers 706 (706 a, 706 b,706 c) is connected to a secondary waveguide pathway 712 (712 a, 712 b,712 c) having a waveguide endpoint 714 (714 a, 714 b, 714 c). In otherembodiments, the secondary waveguide pathways 712 may be omitted and thesecond side of the fiber couplers 706 may include the waveguideendpoints 714. In some embodiments, the length of the secondarywaveguide pathways 712 may vary. For instance, the lengths of secondarywaveguide pathways 712 may be substantially shorter than illustratedsuch that waveguide endpoints 714 are positioned (directly) adjacent tofiber couplers 706.

The light from light sources 702 can be directed out of endpoints 714and towards a retroreflector 716 (e.g. a spherical retroreflector or“cat's eye,” for example). In contrast to the spherical retroreflectors523, 623 discussed above, the retroreflector 716 may be modulated withrespect to the any property of coherent light that can be modulated tofacilitate distance measurements. Non-limiting examples can includemodulating phase, frequency, amplitude, polarization, or any combinationthereof. Such techniques of modulation are well known in the art, andany such device is contemplated. As a result of the modulation of theretroreflector 716, the light reflected by the retroreflector 716 willbe modulated accordingly.

In the illustrated embodiment, for instance, light source 702 a mayproduce light having a frequency f₁ and the retroreflector 716 maymodulate the light at a frequency fm so that it is reflected back inharmonics with frequencies of f₁±fm, f₁±2fm, f₁±3fm, etc. Similarly,light sources 702 b, 702 c may produce light having frequencies f₂, f₃,respectively, and the retroreflector 716 may modulate the light so thatit is reflected with frequencies f₂±fm, f₂±2fm, f₂±3fm, etc. and f₃±fm,f₃±2fm, f₃±3fm, etc. It will be appreciated that fewer or more harmonicfrequencies may be produced and/or detected. In some embodiments, onlythe first two harmonics (f±fm, f±2fm) are used.

In any event, at least some of the reflected light can be reflected backinto the pathways 712 via endpoints 714. Additionally, as noted above, aportion of the original light in each pathway 712 is not emitted atendpoints 714, but is reflected back from endpoints 714. Accordingly,the components of light propagating back through pathway 712 a mayinclude frequencies of f₁, f₁±fm, and f₁±2fm (indicated in FIG. 7A withthe arrow pointing towards photodetector 720 a). Similarly, thecomponents of light propagating back through pathways 712 b and 712 cmay include frequencies of f₂, f₂±fm, and f_(2±2)fm, and f₃, f₃±fm, andf₃±2fm, respectively (indicated in FIG. 7A with the arrows pointingtowards photodetectors 720 b, 720 c, respectively). The frequencies ineach pathway will co-propagate back through the respective pathways 712and fiber couplers 706.

As noted above, the first side of each fiber coupler has two pathwaysconnected thereto. In addition to the initial pathways 704, the fibercouplers also have pathways 718 (718 a, 718 b, 718 c) coupled thereto.At least portions of the co-propagating components of light may bedirected through the associated pathways 718 to photodetectors 720 (720a, 720 b, 720 c). As discussed above, when the co-propagating componentsof light reach the associated photodetectors 720, the beams will producemodulated signals in the photodetectors 720 at frequencies fm, 2fm, 3fm,etc. Such signals can be used to measure distance as discussed above.

For instance, the distance between each endpoint 714 and theretroreflector 716 may be determined. When the distance between three ormore endpoints 714 and the retroreflector 716 are known, the position ofthe retroreflector 716 may be determined by triangulation. In someinstances, the position of the retroreflector 716 may move. Changes inthe reflected light and the photodetector signals can be used todetermine magnitude and direction of the change in position of theretroreflector 716. For instance, changes in the amplitude of thephotodetector signals can indicate movement of the retroreflector 716(e.g., whether the amplitude goes up or down and by how much can be usedto determine the change in position of the retroreflector 716). Thechanges in position, including directions, can be determined byphotodetector signals because the photodetector signals from themodulated light in the adjacent harmonics (e.g., fm, 2fm) are inquadrature. Furthermore, so long as the retroreflector 716 stays withinthe emitted light from the end points 714, the position of theretroreflector 716 can be determined.

FIG. 7B illustrates an example interferometry device 750 that issubstantially similar or identical to the interferometry device 700.Accordingly, the following discussion of FIG. 7B will focus on thoseaspects of device 750 that are different from device 700.

As can be seen in FIG. 7B, rather than having a single retroreflector(similar to retroreflector 716 in FIG. 7A), FIG. 7B illustrates a rigidobject 752 that has a plurality of retroreflectors 754 (754 a, 754 b,754 c) mounted thereon. Each of the retroreflectors 754 may be modulatedto modulate one or more characteristics of light directed thereon. Forinstance, some or all of the retroreflectors 754 may modulate the phase,frequency, amplitude, or polarization of light directed thereon. In someembodiments, retroreflectors 754 may modulate different characteristicsof light or modulate the same characteristics of light to differentdegrees or at different frequencies. For instance, one retroreflectors754 may modulate phase, another may modulate frequency, and another maymodulate amplitude. In other embodiments, each retroreflector 754 maymodulate phase or frequency, but at different rates for frequencies. Forease of reference, retroreflectors 754 a, 754 b, 754 c will be describedas modulating light at frequencies fm1, fm2, fm3, respectively.

In any event, the light emitted from each endpoint 714 may be modulatedby each of the retroreflectors 754 and directed back to each endpoint714. The reflected light may enter the endpoints 714 and co-propagate tothe photodetectors along with the original, unmodulated light that wasreflected from the endpoints 714. Accordingly, each photodetector 720may receive a beam made of the original light and components that weremodulated and reflected back by the retroreflectors 754. For instance,the photodetector 720 a may receive a beam of light having componentsf₁±2fm1, f₁±fm2, f₁±2fm2, f₁±fm3, f₁±2fm3 (indicated in FIG. 7B with thearrow pointing towards photodetector 720 a). Similarly, photodetectors720 b and 720 c may each receive a beam of light having similarcomponents. However, photodetector 720 b will have a component f₂instead of component f₁ and photodetector 720 c will have a component f₃instead of components f₁ or f₂.

These beams can be used to determine up to nine separate distances(e.g., between endpoint 714 a and each of retroreflectors 754 a, 754 b,754 c; between endpoint 714 b and each of retroreflectors 754 a, 754 b,754 c; and between endpoint 714 c and each of retroreflectors 754 a, 754b, 754 c). These distances can be used to determine the position of theretroreflectors 754 a, 754 b, 754 c, and by extension, the position (x,y, z) and orientation (pitch, yaw, roll) of the rigid object 752.

Attention is now directed to FIG. 8A, which illustrates an exampletwo-stage interferometry device 800 that can be used to scan typicallylarger areas than the interferometry devices discussed above. Theillustrated device 800 includes a first stage 802 and a second stage804. The first stage 802 is similar in many respects to device 750discussed above. For instance, the first stage 802 includes a pluralityof light sources 806 (806 a, 806 b, 806 c) that generate coherent lightalong a plurality of associated initial waveguide pathways 808 (808 a,808 b, 808 c). The initial waveguide pathways 808 (808 a, 808 b, 808 c)direct the light from the light sources 806 (806 a, 806 b, 806 c) intoassociated fiber couplers 810 (810 a, 810 b, 810 c). In the illustratedembodiment, the fiber couplers 810 (810 a, 810 b, 810 c) are 2-by-1fiber couplers, meanings that each fiber coupler 810 has two waveguidepathways connected to a first side thereof and one waveguide pathwayconnected to a second side thereof. The initial waveguide pathways 808(808 a, 808 b, 808 c) are connected to the first sides of the associatedfiber couplers 810 (810 a, 810 b, 810 c). The second side of each of thefiber couplers 810 (810 a, 810 b, 810 c) is connected to a secondarywaveguide pathway 812 (812 a, 812 b, 812 c) having a waveguide endpoint814 (814 a, 814 b, 814 c). In other embodiments, the lengths of thesecondary waveguide pathways 812 may vary or be omitted. For instance,the lengths of secondary waveguide pathways 812 may be substantiallyshorter than illustrated such that waveguide endpoints 814 arepositioned (directly) adjacent to fiber couplers 810 or such that thesecond side of the fiber couplers 810 may include the waveguideendpoints 814.

In the illustrated embodiment, the endpoints 814 are connected to areference plate 816. In some embodiments, the reference plate 816 may bea rigid plate having a known position and orientation. In otherembodiments, the end points 814 may be connected to a plurality ofreference plates. For instance, each end point 814 may be connected toits own reference plate. In still other embodiments, the endpoints 814may not be connected to a reference plate, but may still have a knownposition and orientation.

In any event, the light from light sources 806 can be directed out ofendpoints 814 and towards a plurality of retroreflectors 818 (818 a, 818b, 818 c). As with retroreflectors 716, 754 discussed above, theretroreflectors 818 may be modulated with respect to the any property ofcoherent light that can be modulated to facilitate distancemeasurements. Non-limiting examples can include modulating phase,frequency, polarization, amplitude, or any combination thereof. Suchtechniques of modulation are well known in the art, and any such deviceis contemplated. As a result of the modulation of the retroreflectors818, the light reflected by the retroreflectors 818 will be modulatedaccordingly. The modulated light returns back into the endpoints 814 andto associated photodetectors 820 (820 a, 820 b, 820 c) in the same orsimilar manner discussed above in connection with FIG. 7B.

In the illustrated embodiment, the retroreflectors 818 are mounted on arigid gimbal plate 822. The gimbal plate 822 can rotate and/or pivotabout one or more axes. As the gimbal plate 822 rotates and/or pivots,the distances between the endpoints 814 and the retroreflectors 818 willchange. These changing distances can be determined in accordance withthe principles described herein. As a result, the orientation and/orposition of the gimbal plate 822 can be determined. Accordingly, thefirst stage 802 can be used to ascertain and/or monitor the positionand/or orientation of the gimbal plate 822 using the nine distancemeasurements between the fiber ends 814 and the retroreflectors 818.

The second stage 804 employs the movement of the gimbal plate 822 toscan larger areas than is typically achievable with interferometrydevices, including those discussed above. As noted above, light emittedfrom an endpoint of only a single fiber may have a divergence half angleof about 0.1 radians. Accordingly, in order to access an area of about 1square meter, the endpoint would have to be positioned about 5 metersaway from the area in order to allow the light to spread out enough tocover the area. In contrast, using a two-stage interferometry device asdiscussed herein, the distance between the gimbal fiber endpoints andthe to-be-scanned area can be dramatically reduced. The extent of thereduction will depend, at least in part, on the range of motion of thegimbal plate 822. Before proceeding further, a description of the secondstage 804 will be provided.

Similar to the first stage 802, the second stage 804 includes aplurality of light sources (not shown) and a plurality of photodetectors(not shown). Each light source and photodetector is associated with awaveguide pathway 824 (824 a, 824 b, 824 c) having an endpoint 826 (826a, 826 b, 826 c). The endpoints 826 are connected to the gimbal plate822 such that movement of the gimbal plate 822 results in movement ofthe endpoints 826. Light from the light sources can be directed out ofthe endpoints and towards a target 828. Modulated light reflected backfrom the target 828 can enter into the endpoints 826 and propagate tothe photodetectors associated with the endpoints 826 (along with areflected portion of the original light from the light sources). Asbefore, the interference signals from the reflected light can be used todetermine the distance between each endpoint 826 and the target 828.

Using triangulation, the 3D position of the target 828 can then bedetermined relative to the gimbal plate 822, and particularly the fiberendpoints 826 thereon. Once the position of the target 828 is knownrelative to the fiber endpoints 826 on the gimbal plate 822, theposition of the target 828 relative to the reference plate 816 can bedetermined using mathematical relations. In particular, the positionand/or orientation of the gimbal plate 822 can be determined using thefirst stage 802. The positions of the endpoints 826 relative to thefixed retroreflectors 818 on the gimbal plate 822 can be known bypre-calibration as discussed elsewhere herein. With that known, as wellas the position of the target 828 relative to the gimbal plate 822, theposition and orientation of the target 828 can be determined relative tothe reference plate 816.

Although not illustrated, it will be appreciated that a lens may bepositioned in front of any of the endpoints 814, 826 (or any of theother endpoints disclosed herein) to reduce the divergence or tocollimate the beams emitted therefrom. Reducing the divergence orcollimating the emitted beams can increase the intensity of the beams onthe targets.

Although FIG. 8A illustrates the target 828 centered with the device 800and as being relatively small, the target 828 may move relative to thedevice 800 and/or may be significantly larger. In such cases, the lightemitted from the endpoints 826 may need to be redirected in order toshine on the target 828 or desired portions thereof. Accordingly, thegimbal plate 822 can rotate and/or pivot about one or more axes in orderto direct the light emitted from endpoints 826 over a larger region inorder to illuminate on a moving target 828 and/or a portion of a largertarget 828.

In some embodiments, the gimbal plate 822 may have a range of motionthat allows the gimbal plate 822 to rotate about one or more axes up toabout 150°. For instance, from the neutral position/orientation shown inFIG. 8A, the gimbal plate 822 may rotate about 75° in either directionabout an axis (for a total of about 150°). In other embodiments, thegimbal plate 822 may rotate about an axis more than 150°, up to about120°, 100°, 90°, 60°, 50°, or 25°, less than 25°, or in any rangebetween any of the foregoing values. For instance, some embodiments, thegimbal plate 822 may rotate about one or more axes up to about 360°. Insuch embodiments, the gimbal plate 822 may be associated with multiplereference plates 816 disposed about the gimbal plate 822. Each of thereference plates may include multiple endpoints 814 that direct light atthe retroreflectors 818 for monitoring or detecting theposition/orientation of the gimbal plate 822. Alternatively, oradditionally, the gimbal plate 822 may include additionalretroreflectors thereon. The additional retroreflectors may be “seen” bythe endpoints 814 on the reference plate 816 when the gimbal plate 822rotates far enough that the endpoints 814 on the reference plate 816cannot “see” the illustrated retroreflectors 818. For instance, thegimbal plate 822 may include or have connected thereto a cylindricalsurface upon which multiple retroreflectors are mounted and which can be“seen” by the endpoints 814 regardless of the degree to which the gimbalplate 822 is rotated. Furthermore, the neutral position of the gimbalplate 822 may not be located at the center of the range of motion.Additionally, the gimbal plate 822 may have the same or different rangesof motion about different axes.

Prior to use of the device 800, various components thereof may bepre-calibrated. For instance, the relative positions of the endpoints814 a-c, retroreflectors 818 a-c, and endpoints 826 a-c may bedetermined. The pre-calibration can be accomplished in a variety ofways. By way of example, and similar to the method discussed above, amoveable fiber can be used to detect the endpoints 814 a-c andretroreflectors 818 a-c and triangulation methods can be employed todetermine that actual positions thereof (including their positionsrelative to one another). The relative positions of the endpoints 814a-c can be determined with a movable fiber emitting a single wavelength.For instance, the movable fiber can be used to determine the position ofone of the endpoints 814 a-c. From there, the movable fiber can be usedto determine the positions of the other endpoint 814 a-c in the samereference frame, such that the relative positions of the endpoints 814a-c can be determined. Alternatively, the relative positions of theendpoints 814 a-c can be determined with one or more movable fibersemploying absolute distance interferometry (e.g., emitting multiplewavelengths).

After calibration, with the positions of the retroreflectors 818 a-cknown, the positions of the endpoints 826 a-c can be determined. Onemethod of determining the positions of the endpoints 826 a-c can includedirecting light from the endpoints 826 a-c onto an object or objectsthat has or have a known fixed position or positions. The gimbal plate822 can then be moved, which will result in changes in the distancesbetween the endpoints 826 a-c and the fixed object. Using the changes inthe distances and the known movements of the retroreflectors 818 a-c(via movement of the gimbal plate 822), the positions of the endpoints826 a-c can be determined by triangulation.

Attention is now directed to FIG. 8B, which illustrates the device 800again. The embodiment of FIG. 8B is the same as the embodiment of FIG.8A except that target 828 is replaced with target 830. As can be seen,target 830 includes a plurality of retroreflectors 832 (832 a, 832 b,832 c). With a plurality of retroreflectors 832, not only can theposition of the target 830 be ascertained, but the orientation of thetarget 830 can also be ascertained, similar to the manner discussedabove in connection with FIG. 7B and the first stage 802 of FIG. 8A(e.g., by determining the distances between each of the waveguideendpoints 826 and each of the retroreflectors 832).

In some embodiments, the target 830 may also include a probe 834 rigidlyattached thereto. The target 830 may be moved so a tip of the probe 834contacts a desired object or a point on an object. The tip of the probe834 may be positioned with a known (pre-calibrated) 3D position relativeto the plurality of retroreflectors 832. Accordingly, by determining theposition and/or orientation of the plurality of retroreflectors 832, theposition and/or orientation of the probe 834 (and particularly the tipthereof) can be ascertained. It will be appreciated that the probe 834,or the tip thereof, may comprise various components, such as a camera,optical scanning device, or other mechanical device.

It will be appreciated that multiple devices 800 could be employed todetermine the position and/or orientation of the target 830 andassociated probe 834. For instance, multiple devices 800 could beemployed to detect the target 830 in the event that the retroreflectors832 a-c are facing away from one device 800 and towards another device800. In other embodiments, the target 800 may include retroreflectors832 disposed around multiple sides thereof so as to be visible by theendpoints 826 a-c regardless of the orientation of the target 830.

As discussed in connection with FIG. 8A, the device 800 as shown in FIG.8B or components thereof may be pre-calibrated. The methods ofpre-calibrating the device 800 or components thereof may be similar oridentical to those described above. In addition to the pre-calibrationsdiscussed above, the positions of the retroreflectors 832 a-c and theprobe 834 can be determined, and particularly relative to one another.In order to determine the positions of the retroreflectors 832 a-c, amovable fiber (with a known location) can be used to detect each of theretroreflectors 832 a-c and triangulation methods can be employed todetermine the positions of each of the retroreflectors 832 a-c. Once thepositions of each of the retroreflectors 832 a-c is known, the relativeposition of the retroreflectors 832 a-c can be determined relative toone another and relative to each of the endpoints 826 a-c.

With the positions of the retroreflectors 832 a-c known, the position ofthe probe 834 can be determined (particularly relative to theretroreflectors 832 a-c). One method for determining the position of theprobe 834 relative to the retroreflectors 832 a-c is to position theprobe in a known location (e.g., in a conical hole) and move theretroreflectors 832 a-c in a way that keeps the position of the probe834 constant. By monitoring the movements of the retroreflectors 832 a-cusing triangulation, the fixed position of the probe 834 can bedetermined using stage 800.

Attention is now directed to FIG. 8C, which illustrates a device 900that is similar to or the same as the device 800 in FIGS. 8A and 8B inmany respects. Accordingly, the following discussion of device 900 willfocus on the aspects of device 900 that are unique compared to thedevice 800 discussed above.

The device 900 can be used to detect the 3D position of a target 902that has a retroreflector 903 thereon. However, the specific method usedto detect the position of the target 902 differs, at least partially,from the methods used in connection with the device 800. Similar to thedevice 800, the device 900 includes a reference plate 816 and a gimbalplate 822 and the associated light sources 806, endpoints 814,retroreflectors 818, photodetectors 820, etc. for determining theposition and orientation of the gimbal plate 822.

In contrast to the device 800, however, the gimbal plate 822 has asingle waveguide pathway 904 having an endpoint 906. The endpoint 906can direct a beam of light with a frequency f₁ towards the target 902(and particularly towards the retroreflector 903 thereon). Between theendpoint 906 and the target 902, the light can pass through a beamsplitter 908. The beam splitter 908 will allow part of the light tocontinue on to the target 902, while redirecting the returning light toa position sensitive photodetector 910.

The portion of the light that is directed to the target 902 willinteract with the retroreflector 903. The retroreflector 903 willmodulate the light (as discussed elsewhere herein (e.g., such that themodulated light has a frequency of f₁±fm, f₁±2fm)) and direct themodulated light back to the beam splitter 908.

The beam splitter 908 will allow a portion of the modulated light topass therethrough and enter back into the endpoint 906. As with previousembodiments, the modulated light entering back into the endpoint 906(along with an internally reflected portion of the original light) willbe directed back to a photodetector 907. The interference of theoriginal light and the modulated reflected light can be used as beforeto determine the distance between the endpoint 906 and the target 902.

Additionally, the beam splitter 908 will direct at least a portion ofthe reflected light to the position sensitive photodetector 910. Theposition sensitive photodetector 910 can determine whether the outgoingbeam is centered on the retroreflector 903. For instance, the positionsensitive photodetector 910 may include a four-quadrant detector thatcan determine displacement (e.g., horizontal, vertical) of the returninglight, which indicates the centeredness of the outgoing light on theretroreflector 903. If the outgoing light is not centered with theretroreflector 903, then the differences of the signals in the fourquadrants of the four-quadrant detector will have non-zero values. Incontrast, if the outgoing light is centered with the retroreflector 903,then the differences of the signals in the four quadrants of thefour-quadrant detector will have zero values.

The position signals produced by the four-quadrant detector can be usedto lock the gimbal plate 822 (and more specifically the light fromendpoint 906) onto the target 902, similar to laser trackers known inthe art. For instance, the position signals produced by thefour-quadrant detector can be communicated to a feedback loop 912. Ifany of the position signals produced by the four-quadrant detector isnon-zero (meaning that the light is not centered on retroreflector 903),then the feedback loop 912 can activate a motor controller 914 that willcontrol one or more motors 916 associated with the gimbal plate 822. Forinstance, if the four-quadrant detector determines that the light is notcentered on the retroreflector 903, the feedback loop 912 (andparticularly the motor controller 914 thereof) can cause the one or moremotors 916 associated with the gimbal plate 822 to reorient the gimbalplate 822 to center the light on the retroreflector 903 (e.g., so thatthe difference signals determined by the four-quadrant detector arezero). In this way, the position sensitive photodetector 910 and thefeedback loop 912 can lock the light so it is centered on theretroreflector 903 in order to track the movements of the target 902. Bydetermining the distance between the endpoint 906 and the target 902from photodetector 907 as well as the orientation of the gimbal plate822, the three-dimensional position of the target 902 can be determined.

In some embodiments, orientation information about the target 902 canalso be obtained. For instance, the target 902 may include three or moreretroreflectors (e.g., retroreflector 903 and two additionalretroreflectors) thereon. The three-dimensional position of the target902 may be determined as described above based on the modulated lightfrom the retroreflector 903). Additionally, the distances between theendpoint 906 and the additional two retroreflectors can be used todetermine the orientation of the target.

It will be noted that this method may also be able to detect certainorientation information about the target. By way of example, if thetarget pivots or rotates towards or away from the endpoint 906, thisnoted method could detect such movements and thereby determine changesin the orientation of the target.

In some embodiments, the systems disclosed herein may be used to measurerelative position or displacement of a target (or retroreflector(s)thereon) relative to the waveguide endpoints. By way of example,referring to the embodiment of FIG. 7A, the relative position of thetarget 716 compared to the endpoints 714 a-c may be determined usingsingle wavelength light sources 702 a-c and fringe counting methodsknown in the art. In particular, as the target moves closer to orfurther from the endpoints, fringe counting methods known in the art canbe employed to determine the relative change in position or displacementof the target.

As is understood, fringe counting methods have certain drawbacks orlimitations. Additionally, in some cases, it may be desirable todetermine the absolute distance between the endpoints and the target.Accordingly, the disclosed embodiments may be used to determine theabsolute distances between the endpoints and the targets. In some cases,as discussed below, the disclosed embodiments may be modified to providethe capability to determine the absolute distance or position of atarget. It will be appreciated that the following methods for measuringabsolute distances are known in the art. Accordingly, only briefdescriptions of these methods are provided. Furthermore, other methodsfor measuring absolute distances may be employed.

One method for determining the absolute distance between an endpoint anda target is by using multiple lights sources with difference wavelengthsfor each endpoint. For instance, referring to FIG. 8A, endpoint 814 amay be associated with at least two light sources (rather than justlight source 806 a). The two light sources may produce light havingwavelengths that are different from one another, although thewavelengths may be only slightly different from one another. Similarly,each of endpoints 814 b, 814 c may be associated with multiple lightsources rather than just light sources 806 b, 806 c. The multiple lightsources associated with each of the endpoints 814 b, 814 c may emitlight having different wavelengths from one another. In addition tohaving multiple light sources associated with each endpoint, eachendpoint may also be associated with multiple photodetectors and awavelength dispersive element such as a diffraction grating or filter(as opposed to individual photodetectors as shown in the figures). Whentwo light sources (that produce light with different wavelengths) areused with each endpoint, the phase difference between the twowavelengths can be used to determine the absolute distance between theendpoint and the target, rather than just the displacement of thetarget.

Another method for determining the absolute distance between an endpointand a target may include using a variable wavelength light source. Thismethod may include scanning the frequency of the light produced by thevariable wavelength light source and measuring the change in phase dueto the frequency change when the target is stationary. With thisinformation, the absolute distance between the endpoint and the targetcan be measured when the target moves.

The embodiments disclosed herein may be used in connection with a broadarray of technologies. For instance, the disclosed embodiments may beused in or as coordinate measuring machines (e.g., to verify parttolerances), CNC and 3d printing machines (e.g., for calibration and/ormonitoring position of machine tool location or parts), lithography,scanning electron microscope (e.g., calibration), precision X,Y,Zstages, wavelength measurement, and monitoring/detectingexpansion/contraction of materials (e.g., during heating or cooling).Additionally, embodiments disclosed herein may also be used to calibratea coordinate measuring machine and 3D positioning systems.

A number of non-limiting examples of the present interferometer systemand associated methods will now be described:

In one example, an interferometry system can include a coherent lightsource operable to generate a beam of coherent light, separate waveguidepathways optically associated with the coherent light source, aphotodetector optically associated with each waveguide pathway, and atransceiving segment optically associated with each waveguide pathway ata location between the coherent light source and the photodetector, eachtransceiving segment being configured to emit an emitted beam ofcoherent light and positioned to receive a received portion of anemitted beam of coherent light emitted from a transceiving segmentoptically associated with a different waveguide pathway, the receivedportion being combined with coherent light from the waveguide pathwayreceiving the received portion to form an optical interference signal,wherein each waveguide pathway is configured to direct a separateoptical interference signal toward a respective photodetector.

In some examples, the coherent light source can include a plurality ofcoherent light sources.

In some examples, each of the plurality of light sources is configuredto emit a beam of coherent light at a unique wavelength.

In some examples, the coherent light source is operable to emit a beamof coherent light having a wavelength of from 400 nm to 2000 nm.

In some examples, each of the separate waveguide pathways comprises asingle mode optical fiber.

In some examples, each of the separate waveguide pathways can include afirst waveguide segment and a second waveguide segment.

In some examples, the first waveguide segment is configured to directthe beam of coherent light to the transceiving segment.

In some examples, the second waveguide segment is configured to directthe optical interference signal to the respective photodetector.

In some examples, the photodetector is a photodiode having a p-njunction, a photodiode having a p-i-n junction, or combination thereof.

In some examples, the system further includes an optical modulatorpositioned to modulate the beam of coherent light directed through oneor more of the separate waveguide pathways.

In some examples, the optical modulator includes a member selected fromthe group consisting of an acousto-optic modulator, an electro-opticmodulator, a magneto-optic modulator, a mechano-optic modulator, aphase-shifter, and a combination thereof.

In some examples, the optical modulator includes a plurality of opticalmodulators optically associated with separate waveguide pathways.

In some examples, the system further includes a beam splitter positionedto split the beam of coherent light into a plurality of component beams,each of which is directed down a separate waveguide pathway.

In some examples, the system further includes a reflective surfacepositioned to direct a reflected portion of the emitted beam toward thetransceiving segment from which the emitted beam is emitted.

In some examples, the reflective surface is a retroreflector.

In some examples, the system further includes a beam splitter positionedto direct a plurality of split emitted beams toward the reflectivesurface.

In some examples, the beam splitter is a polarizing beam splitter.

In some examples, the system further includes a quarter wave platepositioned to manipulate the split emitted beams.

In some examples, the system further includes a lens positioned to focusthe split emitted beams toward the reflective surface.

In some examples, the system further includes a lens positionedproximate to a plurality of transceiving segments to direct thereflected portion into the plurality of transceiving segments.

In some examples, the system further includes a lock-in amplifieroperatively coupled to one or more photodetectors.

In some examples, a separate lock-in amplifier is operatively coupled toeach photodetector.

In some examples, a method of determining a distance between a pluralityof points can include directing a beam of coherent light along separatewaveguide pathways toward a photodetector optically associated with eachseparate waveguide pathway, each waveguide pathway further comprising atransceiving segment optically associated therewith, emitting an emittedbeam from transceiving segments in separate waveguide pathways,receiving a received portion of the emitted beam at a transceivingsegment optically associated with a different waveguide pathway fromwhich the emitted beam was emitted, and wherein the received portion iscombined with coherent light in the waveguide pathway receiving thereceived portion to form an optical interference signal, deliveringseparate optical interference signals to respective photodetectors togenerate a local photocurrent at each respective photodetector, andrelating a difference between the local photocurrents at eachphotodetector to a distance between the transceiving segments of theseparate waveguide pathways.

In some examples, directing includes splitting the beam of coherentlight into separate component beams and directing each component beamalong separate waveguide pathways.

In some examples, the beam of coherent light includes a plurality ofbeams of coherent light generated from separate coherent light sources.

In some examples, each of the plurality of beams of coherent light has aunique wavelength.

In some examples, the method further includes modulating the beam ofcoherent light in at least one of the separate pathways.

In some examples, the method further includes directing the emitted beamtoward a reflective surface to form a reflected portion of the emittedbeam that is directed toward the same transceiving segment from which itis emitted, a transceiving segment of a separate pathway, or both.

In some examples, the method further includes splitting the emitted beamand directing a split emitted beam toward the reflective surface.

In some examples, the method further includes polarizing the emittedbeam to form a polarized split emitted beam.

In some examples, the method further includes focusing the emitted beamtoward the reflective surface using a lens.

In some examples, the method further includes directing the reflectedpotion of the emitted beam toward a plurality of transceiving segmentsusing a lens.

In some examples, each separate waveguide pathway comprises a firstwaveguide segment and a second waveguide segment.

In some examples, the beam of coherent light is directed to theassociated transceiving segment via the first waveguide segment.

In some examples, the second waveguide segment is configured to directthe optical interference signal to the respective photodetector.

In some examples, at least one photodetector is a referencephotodetector having a fixed position.

In some examples, the local photocurrent is detected using a lock-inamplifier.

In some examples, the lock-in amplifier includes a separate lock-inamplifier at each photodetector.

In some examples, an interferometry system comprising: a plurality ofcoherent light sources, each light source of the plurality of lightsources being operable to generate a beam of coherent light; separatewaveguide pathways optically associated with each coherent light sourceof the plurality of coherent light sources, each separate waveguidepathway having an endpoint configured to emit at least a portion of thebeam of coherent light from the associated light source; a plurality ofphotodetectors, at least one photodetector of the plurality ofphotodetectors optically associated with each waveguide pathway; and aretroreflector configured to receive the light emitted from each of theendpoints, modulate the received light, and direct the modulated lightback to the endpoints, wherein each of the waveguide pathways directs anoptical interference signal to the associated photodetector, the opticalinterference signal being formed of the modulated light received by theendpoint of the waveguide pathway and a portion of the coherent lightfrom the waveguide pathway receiving the modulated light, wherein eachwaveguide pathway is configured to direct a separate opticalinterference signal toward a respective photodetector.

In some examples, each of the plurality of light sources is configuredto emit a beam of coherent light at a unique wavelength.

In some examples, the coherent light source is operable to emit a beamof coherent light having a wavelength of from 400 nm to 1000 nm.

In some examples, each of the separate waveguide pathways comprises afiber coupler.

In some examples, each fiber coupler comprises a first side and a secondside, the first side having two waveguide pathways connected thereto,and the second side having one waveguide pathway connected thereto.

In some examples, each coherent light source is optically coupled to afirst waveguide pathway of the two waveguide pathways and eachphotodetector is optically coupled to a second waveguide pathway of thetwo waveguide pathways.

In some examples, the one waveguide pathways connected to the secondsides of the fiber couplers comprises the endpoints.

In some examples, the retroreflector is configured to phase modulate thelight emitted from each of the endpoints.

In some examples, the retroreflector comprises a plurality ofretroreflectors mounted on a rigid object.

In some examples, each of the plurality of retroreflectors is configuredto receive the light emitted from each of the endpoints, modulate thereceived light, and direct the modulated light back to the endpoints,wherein each of the waveguide pathways directs an optical interferencesignal to the associated photodetector, the optical interference signalbeing formed of the modulated light from each of the plurality ofretroreflectors and received by the endpoint of the waveguide pathway.

In some examples, a method of determining a position of an object,comprises: directing first, second, and third beams of coherent lightalong respective first, second, and third waveguide pathways; emitting aportion of the first, second, and third beams of coherent light fromrespective first, second, and third endpoints associated with therespective first, second, and third waveguide pathways; receiving theemitted portions of the first, second, and third beams of coherent lightat a retroreflector; modulating the emitted portions of the first,second, and third beams of coherent light to form first, second, andthird modulated beams; directing the first modulated beam and a portionof the first beam of coherent light to a first photodetector to generatea first photocurrent; directing the second modulated beam and a portionof the second beam of coherent light to a second photodetector togenerate a second photocurrent; directing the third modulated beam and aportion of the third beam of coherent light to a third photodetector togenerate a third photocurrent; and relating a difference between thefirst, second, and third photocurrents at the first, second, and thirdphotodetectors to a distance between each of the first, second, andthird endpoints and the retroreflector.

In some examples, directing the first modulated beam and a portion ofthe first beam of coherent light to a first photodetector comprisesdirecting the first modulated beam and the portion of the first beam ofcoherent light align the first waveguide pathway.

In some examples, modulating the emitted portions of the first, second,and third beams of coherent light comprises modulating at least one ofthe phase, frequency, or amplitude of the first, second, and third beamsof coherent light.

In some examples, receiving the emitted portions of the first, second,and third beams of coherent light at a retroreflector comprisesreceiving each of the emitted portions of the first, second, and thirdbeams of coherent light at a plurality of retroreflectors.

In some examples, modulating the emitted portions of the first, second,and third beams of coherent light to form first, second, and thirdmodulated beams further comprises modulating the emitted portions of thefirst, second, and third beams of coherent light to form fourth, fifth,sixth, seventh, eighth, and ninth modulated beams.

In some examples, a two-stage interferometry system, comprises: a firststage, comprising: a plurality of coherent light sources, each lightsource of the plurality of light sources being operable to generate abeam of coherent light; separate waveguide pathways optically associatedwith each coherent light source of the plurality of coherent lightsources, each separate waveguide pathway having an endpoint configuredto emit at least a portion of the beam of coherent light from theassociated light source; a plurality of photodetectors, at least onephotodetector of the plurality of photodetectors optically associatedwith each waveguide pathway; a gimbal plate selectively movable aboutone or more axes; and a plurality of retroreflectors mounted on thegimbal plate and configured to receive the light emitted from each ofthe endpoints, modulate the received light, and direct the modulatedlight back to the endpoints, wherein each of the waveguide pathwaysdirects an optical interference signal to the associated photodetector,the optical interference signal being formed of the modulated lightreceived by the endpoint of the waveguide pathway and a portion of thecoherent light from the waveguide pathway receiving the modulated light.The second stage comprises: a second plurality of coherent lightsources, each light source of the second plurality of light sourcesbeing operable to generate a beam of coherent light; separate waveguidepathways optically associated with each coherent light source of thesecond plurality of coherent light sources, each separate waveguidepathway having an endpoint configured to emit at least a portion of thebeam of coherent light from the associated light source of the secondplurality of light sources, the endpoints of the separate waveguidepathways of the second stage being connected to the gimbal plate suchthat movement of the gimbal plate moves the endpoints connected thereto;and a second plurality of photodetectors, at least one photodetector ofthe second plurality of photodetectors optically associated with eachwaveguide pathway of the second stage.

In some examples, the first stage further comprises a stationaryreference plate to which endpoints of the first stage waveguide pathwaysare connected.

In some examples, movement of the gimbal plate about the one or moreaxes changes the direction in which light is emitted from the endpointsof the second plurality of waveguide pathways.

In some examples, the second stage further comprises a second pluralityof retroreflectors mounted on rigid object and configured to receive thelight emitted from each of the endpoints of the second plurality ofwaveguide pathways, modulate the received light, and direct themodulated light back to the endpoints of the second plurality ofwaveguide pathways, wherein each of the second plurality of waveguidepathways directs an optical interference signal to the associatedphotodetector, the optical interference signal being formed of themodulated light received by the endpoint of one of the second pluralityof waveguide pathways and a portion of the coherent light from thewaveguide pathway receiving the modulated light.

While the forgoing examples are illustrative of the specific embodimentsin one or more particular applications, it will be apparent to those ofordinary skill in the art that numerous modifications in form, usage anddetails of implementation can be made without departing from theprinciples and concepts articulated herein. Accordingly, no limitationis intended except as by the claims set forth below.

What is claimed is:
 1. An interferometry system, comprising: a pluralityof light sources, each light source of the plurality of light sourcesbeing operable to generate a beam of coherent light; separate waveguidepathways optically associated with each light source of the plurality oflight sources, each separate waveguide pathway having an endpointconfigured to emit at least a portion of the beam of light from theassociated light source; a plurality of photodetectors, at least onephotodetector of the plurality of photodetectors optically associatedwith each waveguide pathway; and a retroreflector configured to receivethe light emitted from each of the endpoints, the light emitted fromeach of the endpoints being modulated either prior to emission from theendpoints or by the retroreflector, the retroreflector being configuredto and direct the modulated light back to the endpoints, wherein each ofthe waveguide pathways directs an optical interference signal to theassociated photodetector, the optical interference signal being formedof the modulated light received by the endpoint of the waveguide pathwayand a portion of the coherent light from the waveguide pathway receivingthe modulated light, wherein each waveguide pathway is configured todirect a separate optical interference signal toward a respectivephotodetector.
 2. The system of claim 1, wherein each of the pluralityof light sources is configured to emit a beam of coherent light at aunique wavelength.
 3. The system of claim 1, wherein at least one of thelight sources is operable to emit a beam of light having a wavelength offrom 400 nm to 5000 nm.
 4. The system of claim 1, wherein each of theseparate waveguide pathways comprises a fiber coupler.
 5. The system ofclaim 4, wherein each fiber coupler comprises a first side and a secondside, the first side having two waveguide pathways connected thereto,and the second side having one waveguide pathway connected thereto. 6.The system of claim 5, wherein each light source is optically coupled toa first waveguide pathway of the two waveguide pathways and eachphotodetector is optically coupled to a second waveguide pathway of thetwo waveguide pathways and wherein the one waveguide pathways connectedto the second sides of the fiber couplers comprise the endpoints.
 7. Thesystem of claim 1, wherein each of the light sources is configured toproduce light with different phases, amplitudes, or frequencies from oneanother or to phase, amplitude, or frequency modulate the light producedthereby.
 8. The system of claim 1, wherein the retroreflector isconfigured to phase modulate the light emitted from each of theendpoints.
 9. The system of claim 1, wherein the retroreflectorcomprises a plurality of retroreflectors mounted on a rigid object. 10.The system of claim 9, wherein each of the plurality of retroreflectorsis configured to receive the light emitted from each of the endpoints,modulate the received light, and direct the modulated light back to theendpoints, wherein each of the waveguide pathways directs an opticalinterference signal to the associated photodetector, the opticalinterference signal being formed of the modulated light from each of theplurality of retroreflectors and received by the endpoint of thewaveguide pathway.
 11. A method of determining a position of an object,comprising: directing first, second, and third beams of light alongrespective first, second, and third waveguide pathways; emitting aportion of the first, second, and third beams of light from respectivefirst, second, and third endpoints associated with the respective first,second, and third waveguide pathways; receiving the emitted portions ofthe first, second, and third beams of light at a retroreflector;modulating the first, second, and third beams of light either prior toemission from the first, second, and third endpoints or modulating theemitted portions of the first, second, and third beams of light at theretroreflector to form first, second, and third modulated beams;directing the first modulated beam and a portion of the first beam oflight to a first photodetector to generate a first photocurrent;directing the second modulated beam and a portion of the second beam oflight to a second photodetector to generate a second photocurrent;directing the third modulated beam and a portion of the third beam oflight to a third photodetector to generate a third photocurrent; andrelating a difference between the first, second, and third photocurrentsat the first, second, and third photodetectors to a distance betweeneach of the first, second, and third endpoints and the retroreflector.12. The method of claim 11, wherein directing the first modulated beamand a portion of the first beam of light to a first photodetectorcomprises directing the first modulated beam and the portion of thefirst beam of light along the first waveguide pathway.
 13. The method ofclaim 11, wherein modulating the emitted portions of the first, second,and third beams of light comprises modulating at least one of a phase,frequency, or amplitude of the first, second, and third beams of light.14. The method of claim 11, wherein receiving the emitted portions ofthe first, second, and third beams of light at a retroreflectorcomprises receiving each of the emitted portions of the first, second,and third beams of light at a plurality of retroreflectors.
 15. Themethod of claim 11, wherein modulating the emitted portions of thefirst, second, and third beams of light to form first, second, and thirdmodulated beams further comprises modulating the emitted portions of thefirst, second, and third beams of light with three retroreflectors tofurther form fourth, fifth, sixth, seventh, eighth, and ninth modulatedbeams.
 16. The method of claim 15, further comprising producing fourth,fifth, sixth, seventh, eighth, and ninth photocurrents with the fourth,fifth, sixth, seventh, eighth, and ninth modulated beams and the first,second, and third beams of light.
 17. The method of claim 16, furthercomprising relating signals associated with the first, second, third,fourth, fifth, sixth, seventh, eighth, and ninth photocurrents at thefirst, second, and third photodetectors to a distance between each ofthe first, second, and third endpoints and each of the threeretroreflectors.
 18. A two-stage interferometry system, comprising: afirst stage, comprising: a plurality of light sources, each beingoperable to generate a beam of light; separate waveguide pathwaysoptically associated with each light source of the plurality of lightsources, each separate waveguide pathway having an endpoint configuredto emit at least a portion of the beam of light from the associatedlight source; at least one photodetector optically associated with eachwaveguide pathway; a gimbal plate selectively movable about one or moreaxes; and a plurality of retroreflectors mounted on the gimbal plate andconfigured to receive the light emitted from each of the endpoints,modulate the received light, and reflect the modulated light back to theendpoints, wherein each of the waveguide pathways is configured todirect an optical interference signal to the associated photodetector,the optical interference signal being formed of the modulated lightreceived by the endpoint of the waveguide pathway and a portion of thelight from the waveguide pathway receiving the modulated light; and asecond stage, comprising: a light source operable to generate a beam oflight; a waveguide pathway optically associated with the light source ofthe second stage, the waveguide pathway having an endpoint configured toemit at least a portion of the beam of light from the light source ofthe second stage, the endpoint of the waveguide pathway of the secondstage being connected to the gimbal plate such that movement of thegimbal plate moves the endpoint connected thereto; a photodetectoroptically associated with the waveguide pathway of the second stage; aretroreflector configured to receive the light emitted from the endpointof the second stage, modulate the received light, and reflect themodulated light back to the endpoint of the second stage, wherein thewaveguide pathway is configured to direct an optical interference signalto the photodetector to determine a distance between the endpoint andthe retroreflector of the second stage, the optical interference signalbeing formed of the modulated light received by the endpoint of thesecond stage and a portion of the light from the waveguide pathway ofthe second stage; and a beam splitter positioned between the endpointand the retroreflector of the second stage, the beam splitter beingconfigured to direct a portion of the modulated light to a positionsensitive photodetector to detect when the portion of the light beamemitted from the endpoint of the second stage is centered on theretroreflector.
 19. The two-stage interferometry system of claim 18,wherein the position sensitive photodetector comprises a four-quadrantdetector.
 20. The two-stage interferometry system of claim 18, furthercomprising one or more motors associated with the gimbal plate, the oneor more motors being configured to reorient the gimbal plate to centerthe portion of the light beam emitted from the endpoint of the secondstage on the retroflector.
 21. The two-stage interferometry system ofclaim 18, wherein the first stage is configured to determine or monitorthe position and/or orientation of the gimbal plate.
 22. The two-stageinterferometry system of claim 18, wherein the second stage comprises aplurality of retroreflectors associated with a target.